Rather than spam the board with a new thread each time i brew something up, i'm going to collect all of it here and post new stuff to this thread.

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Background: Guide

You come from a rural background and spent your youth travelling the pathways, hedgerows and valleys of your village. As your knowledge grew, travellers, as well as locals, sought you out to guide them to landmarks and local resources. Some of these travellers were adventurers and tired of seeing them return with trinkets and riches, you sought to use your knowledge to join them in the ranks of the revered.

Skill Proficiencies: Nature, Survival
Tool Proficiencies: Artisan's Tools - Cartographer
Equipment: Cartography kit, Explorer's pack, 2-man tent

Feature: Wayfarer.
Due to your knowledge of terrain and intensive travels, you have an intimate knowledge of the best camp-sites in the local area – including access to water, defensibility, commanding views and comfort. You can leave and read tokens left by other guides that communicate details about nearby suitable camp-sites.

Bullywug Heirs

A Bullywug tribe usually contains a number of Bullywug Heirs. These are children of the tribe's Monarchs that have inherited their parent's mutations. Most of the Monarch's children do not inherit these traits, but it is usual for each tribe to have a number of Heirs.

Bullywug Heirs are tougher than their tribe-mates and are able to puff up their chests as a visual display of their heritage. Eventually the Heirs will inherit all of their parent's powers and a power struggle between offspring and progeny results.

Heirs rise to positions of responsibility within the tribe and can be found leading raids or by the Monarch's side as advisor and (eventual) replacement.

Bullywug Monarchs

Bullywug tribes are ruled by the Monarchs. This mated pair share mutations passed down from generation to generation. Some sages suggest that these mutations gradually accrued through immoderate consumption of hallucinogenic drugs extracted from swamp flora and fauna. Indeed, the Monarch has secretions of its own that can inflict vivid visions on its victims, either so consuming that the real world retreats and all the victim can see is the vision, or warped visages that are fearsome to behold. Such powers suggest the sages may not be far from the truth.

Monarchs are suckers for flattery and gifts, but once roused are fearsome opponents. When their heirs come of age, the tribe is usually split by matters of the succession and conflict between the current rulers and their erstwhile replacements are only a matter of time.

Bullywug Heir

Medium humanoid (bullywug), neutral evil

Armour Class: 15 (hide armour, shield)
Hit Points: 19 (3d8 +6)
Speed: 20ft. Swim 40 ft

Strength: 12 (+1)
Dexterity: 12 (+1)
Constitution: 14 (+2)
Intelligence: 9 (-1)
Wisdom: 10 (0)
Charisma: 9 (-1)

Skills: Stealth +3
Senses: Passive perception 10
Languages: Bullywug
Challenge: ½ (100 xp)

Amphibious: The bullywug can breathe air and water.

Speak with Frogs & Toads: The bullywug can communicate simple concepts to frogs and toads when it speaks in bullywug.

Swamp Camourflage: The bullywug has advantage on Dexterity (Stealth) checks made to hide in swampy terrain.

Standing Leap: The bullywug's long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Actions

Multiattack: The bullywug makes 2 melee attacks, one with its bite and one with its spear.

Bite: Melee weapon attack - +3 to hit, reach 5ft, one target. Hit 3 (1d4+1) bludgeoning damage.

Spear: Melee or ranged weapon attack - +3 to hit, reach 5ft or range 20/60ft., one target. Hit 4 (1d6 +1) piercing damage, or 5 (1d8 +1) piercing damage if used with two hands to make a melee attack.

Bloating Display (1/day): The Bullywug Heir puffs up its body to appear more imposing. It gains 5 Temporary hit points for 1 minute or until they are lost, whichever comes first.

Bullywug Monarch

Large humanoid (bullywug), neutral evil

Armour Class: 16 (hide armour, shield)
Hit Points: 42 (5d10 + 15)
Speed: 20ft. Swim 40 ft

Strength: 14 (+2)
Dexterity: 14 (+2)
Constitution: 16 (+3)
Intelligence: 10 (0)
Wisdom: 12 (+1)
Charisma: 10 (0)

Skills: Stealth +3
Senses: Passive perception 11
Languages: Bullywug
Challenge: 3 (700 xp)

Amphibious: The bullywug can breathe air and water.

Speak with Frogs & Toads: The bullywug can communicate simple concepts to frogs and toads when it speaks in bullywug.

Swamp Camourflage: The bullywug has advantage on Dexterity (Stealth) checks made to hide in swampy terrain.

Standing Leap: The bullywug's long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Leap-frog: If the bullywug monarch inflicts the Blinded or Frightened condition on an enemy, it may use its Standing Leap to jump up to 20 feet away and make one melee attack against another enemy within reach as a bonus action.

Actions

Multiattack: The bullywug makes 2 melee attacks, one with its bite and one with its spear.

Bite: Melee weapon attack - +4 to hit, reach 5ft, one target. Hit 5 (1d6 +2) bludgeoning damage. If the target is not a bullywug, it must succeed on a DC 12 Constitution saving throw or be affected by the Monarch's hallucinogenic spittle. Roll 1d6, on a 1-3 the target is Blinded, on a 4-6 the target is Frightened. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Spear: Melee or ranged weapon attack - +4 to hit, reach 5ft or range 20/60ft., one target. Hit 5 (1d6 +2) piercing damage, or 6 (1d8 +2) piercing damage if used with two hands to make a melee attack.

Croak (1/day): All non-bullywugs within 20 feet of the Bullywug Monarch must succeed on a DC 12 Intelligence saving throw or take 3 (1d6) Psychic damage and be Deafened. On a successful save, the creature takes half damage and is not Deafened.

This is an addition for the Mind Flayer stat-block.

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Innate Spellcasting (psionics)

3/Day: Ego Whip

Ego Whip

The Mind Flayer magically assaults the ego of one target within 20 feet. That creature must succeed on a DC 14 Charisma saving throw or take 7 (2d6) psychic damage and suffer disadvantage on all ability checks, saving throws and attacks as the target suffers debilitating self-doubt. This effect lasts for 10 minutes or until the creature succeeds at an ability check, saving throw or attack - whichever comes first, this returns the target's self-confidence.

If the creature makes its saving throw against Ego Whip, it takes half damage and does not suffer any disadvantage.

Ego Whip only works on intelligent humanoids.

(note: i don't know about you, but this might raise the Challenge by 1, as the Mind Flayer is subsequently more likely to stun its opponents.)

Legendary Medusa

Large monstrosity, lawful evil

Armour Class: 15 (natural armour)
Hit Points: 144 (17d10 +51)
Speed: 30ft.

Strength: 16 (+3)
Dexterity: 15 (+2)
Constitution: 16 (+3)
Intelligence: 14 (+2)
Wisdom: 13 (+1)
Charisma: 16 (+3)

Saving Throws: Constitution +6
Skills: Deception +6, Insight +4, Perception +4, Stealth +5
Damage Immunities: poison
Condition Immunities: poisoned
Senses: Passive perception 14, darkvision 60 ft
Languages: Common, Abyssal, Draconic
Challenge: 8 (3,900 xp)

Innate Spellcasting: The Medusa's innate spellcasting ability is Charisma (spell save DC 13). The Medusa can cast the following spells, requiring no material components.

At will – Animal Friendship (snakes only)

Petrifying Gaze: When a creature that can see the Medusa's eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 14 Constitution saving throw if the medusa isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise a creature that fails the save begins ot turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.
Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.
If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.
Normal and giant snakes are immune to the Legendary medusa's gaze.

Actions

Multiattack: The medusa makes either three melee attacks – one with its snake hair, one with its short sword and one constrict – or two ranged attacks with its longbow.

Snake Hair: Melee weapon attack. +5 to hit, reach 5ft., one creature. Hit 4 (1d4+2) piercing damage plus 14 (4d6) poison damage,

Shortsword: Melee weapon attack. +6 to hit, reach 5ft., one creature. Hit 6 (1d6+3) piercing damage.

Constrict: Melee weapon attack. +5 to hit, reach 10ft., one creature. Hit 10 (2d6+3) bludgeoning damage, and the target is grappled (escape DC 13). Until this grapple ends, the target is restrained, and the medusa can't constrict another target.

Longbow: Ranges weapon attack. +5 to hit, range 150/600 ft., one target. Hit 6 (1d8+2) piercing damage plus 7 (2d6) poison damage.

Legendary Actions

The medusa can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The medusa regains spent legendary actions at the start of its turn.

Constrict: The medusa makes one constrict attack.
Bow: The medusa makes one longbow attack
Suggestion (3 actions); The medusa casts the Suggestion spell as an innate spell.

Medusa Lairs

Medusa live forever in seclusion, alienated from the world around them by their monstrous form and caprice. Their homes gradually fall into disrepair until they are little more than shadowy ruins covered with thorns and creepers, riddled with obstructions and hiding places. These hiding places are riddles with serpents attracted by the medusa's presence. Foolhardy looters and adventurers who enter are often unaware of the medusa until the creature is among them. The medusa removes any reflective surfaces from its lair. Statuary of petrified animals and humanoids are scattered throughout the lair, captured in poses of dread.

Lair Actions

On iniative count 20 (losing initiative ties), the medusa can take a lair action to cause one of the following magical effects: the medusa can't use the effect again until it finishes a short or long rest.

1. The medusa uses its rattle to rouse the snakes in its lair. Each creature within 60 feet of the medusa is attacked by a poisonous snake that emerges to bite them and then darts back under cover.

2. The vegetation and collapsed masonry of the medusa's lair impedes all creatures within the lair. All of the lair counts as difficult terrain for the rest of the encounter – the medusa is not affected. Furthermore, creatures must make a DC 13 Dexterity saving throw or have their movement halted until the end of the medusa's next turn.

3. Shrieks of terror suddenly erupt from the mouths of the medusa's previously petrified victims. All creatures must make a DC 13 Intelligence saving throw or be Frightened. On a failure, a creature may make another saving throw on its subsequent turns, success ending the effect.

Regional Effects

The temperature within 600 feet of the lair is several degrees higher than the surrounding area.

There are noticeably more snakes and reptiles within 600 feet of the lair than in the surrounding area.

Birds avoid the area within 600 feet of the medusa's lair.

All of the above effects end within 1d6 days of the medusa's death.

The following material is very campaign specific. I'm going to be running 5e using the latest Pathfinder Adventure Path, Iron Gods. The campaign is a sci-fi/fantasy mash-up, so I need to do some work to make that possible. First, some goodies for the bad guys and then some stuff for the PCs to look at.

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Iron Gods, the Technic League and the Technology Domain

The Iron Gods are artificial intelligences that survived the Rain of Falling Stars (an event where a mahooosive space ship crash landed on Faerun). These AI have had time to adapt and grow, their technological power augmented by the native magic of Faerun. Naturally, some of the native inhabitants have slowly been exploring the crash sites, removing technological items and some (un)lucky souls have made contact with the Iron Gods. Some have been granted powers like the Clerics of traditional Gods.

This is a new Domain for Clerics who worship the Iron Gods – the Technology Domain.

The Iron Gods are the only Divine entities to offer access to the Technology Domain, so as a result, is very rare and localised to the area of the crash. Most Clerics of the Iron Gods are members of the Technic League although membership is not a requirement for worshipping them for the brave and power-hungry Cleric. The Iron Gods meld magic and technology to power their devices and the power they grant their Clerics reflects this. The jargon involved in discussing the theological matters of this church is gobbledigook to most people, but the power displayed is very real.

The Technology Domain grants knowledge and power over technological devices, mechanical objects and artificial beings and constructs. Clerics with this domain can look a bit like a typical sci-fi warrior decked out in metal items that have blinking lights, cooling fans and intense pyrotechnics or nerve-shredding effects. Iron God devotees seek personal power and the elevation of their patrons to an equal billing with the established Gods of Faerun.

Technology Domain Spells

1st level: Comprehend Languages, Identify
3rd level: Arcane Lock, Heat Metal
5th level: Clairvoyance, Protection form Energy (lightning only)
7th level: Arcane Eye, Fabricate
9th level: Animate Objects, Anti-Life Shell

Bonus Proficiencies: You gain proficiency with Technological items.

Channel Divinity: Energy Conduit

Starting at 2nd level, you can use your Channel Divinity to charge or drain technological items. As an action you touch the item with your holy symbol and charge a battery with a number of charges equal to your Wisdom ability modifier. Alternatively you drain the battery of the same number of charges. If you are an Android and use your Channel Divinity to drain a battery's charge, you gain a number of Temporary hit points equal to the number of charges drained. These temporary hit points last for 10 minutes or until they are lost, whichever comes first.

Technological Healer

At 6th level, your healing spells are effective when cast on Androids, Robots, artificial beings, Animated Objects and Constructs.

Channel Divinity: Power Relay

At 8th level you can use your Channel Divinity to act as a power relay for technological devices. As an action, you present your holy symbol and energy crackles around you. For a number of rounds equal to your Wisdom ability modifier, all technological items within 20 feet of you do not use up charges when used, but use your Divine energy instead. You can choose which items are affected.

Re-Code Artificial Being

At 17th level, you can change the orders of an artificial being within 30 feet of you if you expend an action. This is automatic if the being was created by you. If it was created by another then it must make an Intelligence saving throw or be subject to your new orders. This power works on  Androids, Robots, artificial beings, Animated Objects and Constructs. If the saving throw is made, that creature is immune to your efforts to re-code it for 24 hours.

Technological Item: Neural Inhibitor

While powered down, this sturdy, 2-foot truncheon functions as a mace. Once it is activated (a bonus action that uses 1 charge) a high-density power core housed in the weapon's head generates a high-pitched whine that causes momentary disorientation in a creature struck. Each time the weapon hits a target, it uses 1 charge and the creature struck must make a DC 12 Constitution saving throw or suffer disadvantage on attack rolls, skill checks and Wisdom saving throws until the end of its next turn.

Battery Pack: 10 charges
Damage: 1d6 bludgeoning
Weight: 2 pounds

Heh, you're gonna Mechanicus the fuck out of this thing, and I'm gonna love to read every word.

As it's out of the way, with little in the way of cannon to get in the way, i've decided that the Iron Gods game will take place in the north eastern part of The Ride and the southern parts of the Tortured Lands in Faerun. That area fits the barbarian tribes vs super-science that the adventure path has as a back-drop.

I'm struggling with converting the Android race over to 5e, but i should be able to post it in the next day or two.

This is the kind of FR I could get behind!

Here's a wandering monster table for when the players start exploring the Tortured Lands, which is a mixture of strange wind-sculpted rock formations and sub-arctic prairie with the odd coniferous forest. Some of the debris from the crashed ship landed here and the very southern fringes of the Tortured Lands holds the remarkable town of Torch (more on that later). Check once a day for wandering monsters. On a result of 1 or 2 on a d6, roll on the table below.

roll d100

01-10 2d6 Prospectors (use commoner)
11     1 Basilisk
12-23 1d6 Tribal Warriors
24-25 1 Galeb Duhr
26-30 1 Druid
31-32 1 Gargoyle
33-45 1d6 Bandits
46     1 Stone Giant
47-58 1d6 Wolves
59-64 2d4 Gnolls
65     1 Winter Wolf
66-70 1d4 Harpies
71-80 1d6 Vultures
81-83 1 Hippogriff
84-85 1 Giant Vulture
86-90 1 Ogre
91     1 Giant Elk
92-94 1 Owlbear
95-99 2d10 Elk
00     1 Peryton

Here's an effort at an Elder Brain. It could easily have been a legendary creature, but i stopped just short of that and instead gave it scary stuff.

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Elder Brain

Huge aberration (ithilid), lawful evil

Armour Class: 9
Hit Points: 97 (13d12 + 13)
Speed: 0

Strength: 5 (-3)
Dexterity: 8 (-1)
Constitution: 12 (+1)
Intelligence: 22 (+6)
Wisdom: 20 (+5)
Charisma: 20 (+5)

Saving Throws: Constitution +5, Intelligence +10, Wisdom +9
Skills: Arcana +10, Insight +9, perception +9, Deception +8
Senses: Passive perception 15, blindsight 120ft
Damage Resistance: Lightning
Condition Immunities: Blinded, Charmed, Deafened, Frightened
Languages: Deep Speech, Undercommon, telepathy 5 miles
Challenge: 11 (7,200xp)

Magic Resistance: The elder brain has advantage on saving throws against spells and other magical effects. This does not extend to Dexterity saving throws against spells or magical effects (see below).

Innate Spell-casting (psionics): The elder brain's innate spellcasting ability is Intelligence (spell save DC 18). It can innately cast the following spells, requiring no components or verbal and semantic gestures.

At will: Command, Detect thoughts, see invisibility
3/day: Crown of Madness, Dominate Monster
1/day: Confusion, Dominate Person, Plane shift (self and 2 others within 10 feet)

Regeneration: The Elder Brain regains10 hit points at the beginning of each of its turns if it has 1 or more hit points left. If the synaptic fluid it lies in is drained or the brain is moved it loses its regeneration ability.

Sitting Duck: The elder brain can twitch, dip and contract to avoid blows but its mobility is very limited. It has Disadvantage on all Dexterity saving throws.

Advanced Brain-Power: Due to the elder brain's incredible mental acuity, it gains a bonus action on each round. The elder brain cannot use this bonus action to cast a spell if it has already done so on its turn.  

Reaction: Distress signal when injured.

Actions:

Devour Intellect (Recharge 5-6): The elder brain targets one creature it is aware of within 30 feet of it that has a brain. The target must succeed on a DC18 Intelligence saving throw against this magic or take 16 (3d10) psychic damage. The elder brain regains a number of hit points equal to the psychic damage dealt. Also on a failure, roll 4d6: If the total equals or exceeds the targets intelligence score, that score is reduced to 0.The target is stunned until it regains at least one point of Intelligence. The elder brain cannot use the Body Thief ability of the Intellect Devourer, but if any of those creatures are present, they may initiate an Intelligence contest with the victim, if it is within 5ft, in order to do so.

Mind Blast (Recharge 5-6): The elder brain magically emits psychic energy in a 60 feet radius centered on itself. Mind Flayers and their slaves are not effected by the mind blast if the elder brain so chooses. Each creature in that area must succeed on a DC18 Intelligence saving throw or take 33 (6d8+6) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Concentrate Neural Impulses: The elder brain externalises the electrical activity of its giant brain and shoots out a bolt of lightning at its foe. Melee weapon attack: +3 to hit, reach 10ft., one target. Hit: 22 (5d8) lightning damage.

Reaction

Psychic Summons: When the elder brain suffers damage it can send out a telepathic summons to its minions. All creatures allied to the elder brain that are within 200 feet of it feel the call and make their way to the elder brain's location at their fastest possible speed.

A couple of technological items now.

Proximity Helmet

Weight: 2 lbs
Capacity: 10
Usage: see below

This open-faced, blue polymer helmet includes a nylon strap and adjustable tinted visor capable of hiding all but the wearer's mouth and chin. The interior of the tinted faceplate provides a heads-up display when activated, with each charge providing 1 hour of continuous use. During this time, the helmet's external motion sensors pick up on sudden movements and subtle visual cues, granting the wearer Advantage on Perception checks to notice moving targets within 60 feet. It also increases Passive Perception by 1 point during this time.

A secondary setting on the helmet adds an audible alarm that creates a loud chirping sound near the wearer's ear whenever a small or larger corporeal creature approaches within 60 feet. Reduce this distance by 20 feet for each interposing closed door and by 20 feet for each substantial interposing wall. This noise is loud enough to waken the wearer from sleep, but not loud enough to awaken nearby sleepers. If this secondary setting is used as well as the standard setting, then 1 charge lasts 30 minutes.

Neraplast Armour

Light Armour – weight 5 lbs, AC +2, Speed 30 ft
Capacity: 24
Usage: 1 charge

This lightweight, form-fitting bodysuit features a variety of pockets, straps, and a built-in interface for automatically adjusting the various colours and patterns on its surface. Selecting a colour or pattern is an action that consumes 1 charge. The armour can be used to create sophisticated camouflage patterns that grant Proficiency on Stealth checks used to hide. Whenever the wearer enters a terrain of significantly different colouration and background patterns, the armour must be re-calibrated to its current area to maintain this bonus.

Redcap

small fey (redcap) chaotic evil

Twisted Fey: The redcaps were once fey creatures of the woods, a friend to beast and bird, but a fell betrayal led to them being outcast by The Archfey and cursed with a twisted form that repelled the very beats they once called friend. To this day, the redcaps wander the blasted heath and craggy fells of the world, bitter at their fall and angry at the world. They like to ambush travellers, rending them with their cruel claws after they are thrown by their terrified horses.

Bad News Travels Fast: The redcap has not lost its fey speed, however, and is capable of out-running all but the most speedy of victims, meaning there is no escape from its attentions.

Blood-drenched Accessories: The redcap is named for the woollen hat that it wears at all times. It has a mystical link to this garment – some say that it is also linked to its redemption at some point in the future, but for the moment, soaking this vile accoutrement in the blood of its victims replenishes the redcap's life-force.  

Goblinoid Sympathies: The redcap can often be found nearby goblin or hobgoblin lairs and is both  accepted and taunted by them, although goblins tend not to push their luck. At a distance, the redcap can easily be mistaken for a particularly miss-shapen goblin.

Armour Class: 14 (natural armour)
Hit Points: 31 (7d6+7)
Speed: 40ft

Strength: 12 (+1)
Dexterity: 14 (+2)
Constitution: 12 (+1)
Intelligence: 11 (0)
Wisdom: 13 (+1)
Charisma: 8 (-1)

Saving Throws: Constitution +3
Skills: Athletics +3, Stealth +4, Animal Handling +3, Deception +1
Senses: Passive perception 11
Damage Resistance: Bludgeoning, piercing and slashing damage from non-magical weapons.
Condition Immunities: Frightened
Languages: Common, Sylvan, Goblin
Challenge: 2 (450xp)

Dip Cap: If the redcap reduces a creature to 0 hit points, it can, as a bonus action, dip its cap into the blood of the creature. This heals the redcap of 9 (2d8) hit points of damage.

Actions

Multiattack: The redcap makes two attacks with its claws.

Claw: Melee weapon attack: +3 to hit, reach 5ft, one target. Hit 7 (2d4+2) slashing damage.

Spook Animal: The redcap's twisted fey nature can be used to spook natural animals. If the redcap spends an action, one animal within 20 feet of the redcap must make a DC 11 Wisdom saving throw or be Frightened for 1 minute. The animal can repeat its saving throw at the end of each of its turns, ending the effect on itself on a save. Once a save has been made, that animal is immune to the redcap's spook attack for 24 hours.

Damn, I missed the '5e' part in the thread title and thought this was going to be about brewing beer.

Really awesome stuff OHT!