How often do your players encounter situations or creatures not in any books?  

For instance in the last session I ran the players ended up fighting unsubstantial snakelike streamers of fairy fire.  It's 5th ed.  There isn't anything like them in the books, and I didn't even think if them until the bard cast fairy fire in the fires room.

There was a black urn that collected necrotic energy.  I decided to have the fairy fire act strangely in the first room to let the know something wierd was going on, then when circumstances conspired so they took the lid off the urn and the energy escaped to reanimate the skeletons they had just destroyed (again) I had it thake the form if snake like streamers of fairy fire which slithered into the bones to reanimate them.

They were supposed to be completely insubstantial but the warlock (with a death god patron) rolled a 20 to hit.  That seems like it should be relevant.  So I made up some conditions under which magic could affect them.  The the wizard critically failed on shocking grasp.  So she ended up pumping that energy into the snake.  It became substantial and bigger, now they could attack it and it could attack them.

Any way.  I winged all of that.  My plans for the creatures changed twice when their was new input and a circumstance I had not for seen.  
How oftern do you put things like that in with no rules?

My players had a great time.  Loved the snakes.  
It was a mix on new players and old.

Two part answer for the different issues brought up in the OP

1. I homebrew about 60% of my creatures I think, usually drawing inspiration from video games. Lately I've been thinking of running a campaign with a much lower number of more frequent and more thoroughly detailed creatures. I've got the Fire Emblem games on my mind, and those use an extremely small pallet of versatile non-humans to pretty good effect, avoiding the kitchen sink feel that's been grating on me.

Trouble is, 5e presumes in its very mechanics that you are going to be using certain creatures, and the books don't flag that fact. For instance, you must have CR 2 and 5 elementals for conjuring spells to work, you must have zombies and skeletons for necromancy to work, you must have fiends and fey and Cthulhu shit for warlock options to work, etc.

2. I try to avoid changing monster stats mid-fight because that strikes me as too close to illusionism and would piss me off if I were in the player seat and figured out the GM was doing it.

Quote from: Shipyard Locked;922297Two part answer for the different issues brought up in the OP

1. I homebrew about 60% of my creatures I think, usually drawing inspiration from video games.

Maybe the same for me, depending on what you call a "homebrewed creature". The bulk of the creatures in any given area will be prosaic. Some creatures will be "unique", but in the sense that they have some quirky add-on. "A goblin who knows a Dimension Door spell". Only a few will stand out as "not something you'd recognize".

Quote from: Shipyard Locked;9222972. I try to avoid changing monster stats mid-fight because that strikes me as too close to illusionism and would piss me off if I were in the player seat and figured out the GM was doing it.

Occasionally, I set it up where a certain feature is randomly determined during play, like a magical creature is resistant to magic of a given type if their first save roll is 4 over the minimum needed. And unexpected uses of magic may call for unexpected results. But yeah, I don't change many stats on the fly.

I play a lot of superhero games so unique is clearly the way to go. Pit 4-5 heroes with unique against battling 4-5 supervillians with just as unique powers, that's were it's at. I find it sad when you find in publish supers adventures mostly stock aliens, robots or henchmen. I get it, stock opponents is less work but it sort of misses the point.

Moving away from superhero games, over the years I've come up with a method for my house systems (Bounty Hunters of the Atomic Wastelands, Cyberblues City) to combine both unique and stock. The way I deal with cannon fodder opponents in these is a bit the MR in Tunnels & Trolls. All critters have two randomly set properties, Rank and Health which are represented by colour-code D6 and one or more free form traits like "Zombies always go last" or "Robots are immune to mental attacks".

I keep a pool of pre-sorted multi-coloured D6 in an opaque jar so when it comes to generate an encounter, I just blindly draw some dice from the jar, what comes out is what they character's encounter.

This system produces wild, unbalanced and unique encounters without having to put any effort. It works particularly well for things like a group of bandits where there is no reason that all the members of the gang are equally experienced and equally tough.

PS: When I was running Gamma World (old TSR version) I used a lot of unique opponents rather than stock critters. I had a really good fan created character generator and with it I could create a click any number of unique mutants. Fun times.

Of course it is pretty shallow - GURPS it is not - but works very well for the games that implement it.

Quote from: Shipyard Locked;922297Trouble is, 5e presumes in its very mechanics that you are going to be using certain creatures, and the books don't flag that fact. For instance, you must have CR 2 and 5 elementals for conjuring spells to work, you must have zombies and skeletons for necromancy to work, you must have fiends and fey and Cthulhu shit for warlock options to work, etc.

Not really. A player or DM can most certainly tweak their "whatevers" cosmetic appearance and theme. Within the rules since 5e gives you a-lot of interpretive leeway as long as you dont chance the foundations. Your elementals could be pokemon. Your skeletons could be phantoms or skeletal  wolves etc. Your patrons could be cosmic forces, Elmer Fudd, a big tree, a litch whatever.

Quote from: Headless;922290How often do your players encounter situations or creatures not in any books?  

...

Any way.  I winged all of that.  My plans for the creatures changed twice when their was new input and a circumstance I had not for seen.  
How oftern do you put things like that in with no rules?

Since I generally homebrew the hell out of whatever system I'm playing, it's very common for me to introduce my own twist of a mythological trope (like fae or dragons), as well as invent some completely unique monsters.  

I very much subscribe to the philosophies of DCC and LotFP with regards to monsters.  Monsters should be unpredictable, and if not always strictly unique, still always memorable.  Just as DCC recommends, I usually describe the monster and let the players name it, unless it's a well-known type (like beastmen and dragons).  Other times, I will have NPCs use labels, but very sloppily.  Something might be called a demon one moment, a creature the next, etc.  For instance, there are vampires, but the term is extremely loose (i.e. a sentient undead, reunited with a body, which must feed on the living in some manner) and may be mistakenly applied to things that don't strictly qualify (like a perfectly living blood-sucking monster).  

The thing I always try to keep in mind is that monsters shouldn't fit into a Linnean taxonomy.  So some are unique, some classes are very broad, and plenty of things are imprecisely classified.

As for how you ran that session, I like how you think on your feet.  IMO that's a great example of rulings-not-rules.  Magic should be unpredictable, like that.

Quote from: Headless;922290Stock creatures or unique ones

Yes.

Oh, and great play report — this sort if improvisation made for some of our most memorable episodes!

Thanks.  

Shipyard should apreaccate that they had to fight the whole final room again cause they went back to take a short rest.  Normally I wouldn't do that, but urn of necrotic energy, and one of the charcters almost died in that room.

In my fantasy (non-D&D TFT & GURPS) games I've usually used a sub-set of the (already much smaller than D&D's) published creatures & races, with a fairly common sprinkling of variations and fresh homebrew creatures. Unless a creature's type is common, well known to the PCs, and clearly identifiable, I remain a bit vague/coy about what they are. I do that with many details of the whole world - I describe what the PCs observe or think they observe, rarely confirming categorically what exactly something is or what it's stats are, etc.

I generally use stock monsters (maybe 95+% just from the AD&D Monster Manual, in fact), but I often alter them in some way.  That's much more common for me than actually just inventing a monster out of the blue.

Most of my "new" monsters are just old ones with a new name, new Achilles heel, maybe a few stats changed. In one case, I wanted a 2 HD humanoid for my quasi-Dark Ages setting to replace the gnoll, which I thought was better suited for an Arabian Nights/Sinbad/Aladdin setting. So I just took most of the gnoll stats but changed the picture to something more goblin-ish, then gave it a new name: Baugan, Bogan, Bogeyman -the offspring of a bugbear mating with a human. They lurk around villages and small towns looking to pick off unwary stragglers.

There are a few I created whole cloth, but mostly I use old cars with a new coat of paint.

Yes.

Quote from: Headless;922290How often do your players encounter situations or creatures not in any books?  

There are always some, but in a way never as I'm 100% homebrew, but I do have my own homebrew book with 100's of creatures.  

QuoteThere was a black urn that collected necrotic energy.  I decided to have the fairy fire act strangely in the first room to let the know something wierd was going on, then when circumstances conspired so they took the lid off the urn and the energy escaped to reanimate the skeletons they had just destroyed (again) I had it thake the form if snake like streamers of fairy fire which slithered into the bones to reanimate them.

ahhh like Pandora's box some players can't resist.

QuoteThey were supposed to be completely insubstantial but the warlock (with a death god patron) rolled a 20 to hit.  That seems like it should be relevant.  So I made up some conditions under which magic could affect them.  The the wizard critically failed on shocking grasp.  So she ended up pumping that energy into the snake.  It became substantial and bigger, now they could attack it and it could attack them.

Any way.  I winged all of that.  My plans for the creatures changed twice when their was new input and a circumstance I had not for seen.  
How oftern do you put things like that in with no rules?

I've done it and it can work out well, always when I haven't completely thought the creature out.  I try to be balanced and logical, not skewing things one way or another, but I always wonder if I am so I avoid it.  My palyers love tough creatures that they can figure out what works on.

Quote from: Skarg;922476... Unless a creature's type is common, well known to the PCs, and clearly identifiable, I remain a bit vague/coy about what they are. I do that with many details of the whole world - I describe what the PCs observe or think they observe, rarely confirming categorically what exactly something is or what it's stats are, etc.

Exactly this.  As I used a skill based system there are a couple skills that overlap to provide knowledge on creatures, in my games this can mean the difference between life and death, not so much creature weaknesses as habits.  Few creatures just attack out of the blue and fight to the death, unless you are seen as a threat or prey.  Information is power.

Quote from: Soylent Green;922316PS: When I was running Gamma World (old TSR version) I used a lot of unique opponents rather than stock critters. I had a really good fan created character generator and with it I could create a click any number of unique mutants. Fun times.

Gamma World is a good game for using fresh monsters. As you have a world bubbling with mutation, it makes sense to be regularly encountering things that have never been seen. There also isn't any need to draw on traditional mythology.