There's Zuggtmoy sealed within the Temple of Elemental Evil, Zargon at the bottom of the Lost City, Lolth within the Demonweb Pits, Acererark at the heart of his house of death, and many more. Several modern & classic adventures end with a climactic battle against a legendary opponent (Boss). While implementation of separate game mechanics for bosses can definitely lean into the combat as sport vs. war debate, I think boss mechanics still work in an OSR paradigm as long as you're willing to risk your work creating the boss be "wasted" by being overcome without combat, such as through negotiation, evasion, or exploiting some vulnerability.
I enjoy designing boss creatures, and I'm putting together a "template" I can slap on a more standard OSR creature to turn it into a boss. With that said, some boss mechanics I've seen for D&D type games (OSR & Modern) include:
Fear (0e-2e). While inconsistent with the exact mechanics, fear effects in old-school d&d generally set a Hit Die threshold, below which creatures would flee in terror upon sight of a monster, like a lich or dragon. This prevents most henchmen from joining the player characters against a climactic foe, or if encountered by weak player characters arguably saves them from their own potential stupidity.
Inflated Defense (1e-5e). In order to allow your boss to survive longer, some inherent defense is inflated. In 1e & 2e, this is often a creature's Armor Class & Immunities (Demlich: 50 hp, AC -6 (26), Immune to almost everything). From 3e onward, the approach is more often to drastically increase a creature's hit points (Demilich: 80 hp, AC 0 (20), not as many immunities, but takes half damage from ALL magic weapons).
Legendary Resistance (5e). Rather than giving a boss a pile of immunities to obnoxious spells & conditions, bosses in 5e have an ablative resistance allowing them to automatically soak up a couple save or die effects. I have mixed feelings on legendary resistance. Even with low saving throw & high magic resistance, I like the idea that there is still a small chance a boss can be instakilled with petrification, poison, or death magic.
Legendary Actions (5e). I think this works better. Giving a single creature the option of acting multiple times in a round, to counter the action economy problem of 4~6 PCs, perhaps 8~12 henchmen too, vs. 1 big monster.
Magic Resistance (1e-2e). Often given to creatures intended to be treated as bosses, a percentage chance that spells simply fail to take effect (May be fixed or variable based off level). Seems to have similar design goal to legendary resistance, reducing the chance that a boss is immediately destroyed by the wizard's slew of save or die effects.
Paragon Creatures (Angry GM). From
Angry GM's blog, the basic idea is you use the game mechanics of two creatures, but in the story of the game call it 1 creature. This handles increased durability through HP, more actions, the creature changes at 1/2 hp (when mechanically one of its components would die).
Recharge (5e, sorta 0e). A powerful ability that can be used, and then in an uncertain period of time can be used again. I actually like this better than the 1e/2e version which usually makes this a 3/day ability, as the optimal tactic as a dragon is often to spam your breath in the beginning of a fight. 0e used both. A dragon could breath 3/day, but would only do so on a 2d6 roll of 7 or more, sorta like the 5e recharge, but flavored as "monster AI."
While there's several more I'm sure any of you could list, my concept for a "Boss Template" for OSR games looks something like:
1. Boss Vitality. The Boss has Max hp/die (assuming 6 or 8 per die, not something crazy like 20). Usually ad&d creatures with hit points listed instead of hit dice already have max hp per die or inflated hp (golems, tarrasque, solar, etc.) so ignore this.
2. Boss Resistance (more than 1/2 hp). The Boss has advantage on saving throws until at 1/2 hp or less.
3. Boss Fury (1/2 hp or less). The Boss can take a 2nd turn in combat while at 1/2 hp or less. This turn, or even separate components of the turn like movement, attacks, etc. is taken at the end of a player's turn.
Then choose one of these two, depending on how much work you want to do:
4. Boss Contingency (Easier). At 1/2 hp or fewer, or maybe under some specific condition, the boss uses one of its non-damaging special abilities or items, like the druid transforms into a T-Rex, the Fighter drinks a potion of giant strength, the cleric heals themself, etc.
4. Boss Escalation (Harder). Something about the boss becomes more threatening as time passes, based in part on the current round count (Usually maxing at round 6, taken from 13th age). For example, zombie minions of the lich pour into the room: 2 on round 1, 4 on round 2, etc. (Max 12). An amazon possessed by a Type V: Marilith Demon sprouts another arm each round, gaining an additional attack. The Waves of heat generated by a furnace golem get hotter & hotter each round (Maybe 1-6 damage/round from heat & flames). Definitely requires the most creativity & work from the GM, but I find it fun.
Ya'll got your own thoughts on how you'd handle this? Should bosses even have separate rules? Useful boss mechanics I didn't include here?