We’ve all been there. You unleash an awesome boss monster on the party, expecting it to last good solid 5-6 rounds, only for it to suffer from premature evisceration. So what do you when your dracolich drops on round 3? You can leave the players unsatisfied, or cheat and pump up its hp, or use this one weird trick.
This idea only applies to D&D and D&D-like games. And it doesn’t mess with any of the math of the system, either!
Double the hit points of your boss. Decide how many rounds you want it to last, the aforementioned 5 or 6 is fairly standard. Divide the original hp by this number to find the boss’ decay, then round it to something easy to use. Finally, give the boss a trait: the first time it is hit in a round, it takes extra damage equal to the decay number. That’s it.
So if you have, say, a Tarrasque with 1200 hp that you want to last at least 6 rounds (a setup with nice, round numbers), give it 2400 hp instead, with decay of 200. If the party is doing as well as you expect them to do, on round six all the extra hit points will be gone, and they will be facing the original 1200 hp Tarrasque, hopefully about to defeat it. If they have unleashed crazy synergies or maybe simply 5 crits in 3 rounds, they’ll kill it on round 5 or maybe 4 instead, the undecayed bonus hp acting as padding. Importantly, good tactics or plain luck will still have mattered.
Once the decay is done and all the bonus hp are gone, you’ll probably want to “switch off” the decay trait – everything is back to normal. Or maybe the boss turned out to be tougher than you thought, and the party actually needs the help decay provides to finish it off.
A possible tweak involves dividing the decay number by 2 or 3, and having the decay trait trigger corresponding number of times per round, but only once per player. This removes the emphasis from landing one attack each round, instead bringing it back to fighting the boss, though I don’t expect this to be an actual issue in play.
This idea is, in a way, a reverse of 13th Age’s escalation die. Whereas the ED guarantees the battle will eventually swing in PC’s favor, boss decay guarantees the swing will not be too abrupt.