False Hydra thoughts - 02. How to put it in your games
As for putting it in your game, a false hydra is a surprisingly easy thing to put in if you do it "correctly" (Ok, there isn't a correct way. There are a million correct ways. This way makes it easy to drop in basically anywhere, especially in a town)
You can start with disappearances or you can just start with the party finding an extra cot after their long rest. They have no idea who it would belong to. They find identifiers, paintings, notes, etc that seem to detail the party's adventures significantly. Whatever it takes to not be some rando. This clearly was a person important to the party, but they don't know who it was.
To me, it's important for the players and party to learn all this at the same time.
A savvy player probably figured out that this is a memory eating creature by now. Maybe even a false hydra. That doesn't matter. It could even be good.
This lost party member left clues behind. Probably a note to the party explaining why they left. Maybe they said they were getting breakfast. Maybe they were following up on a lead. Maybe there isn't a note, but the party is missing some part of their morning routine - they crave something, but they don't know who did it. These are aspects that they don't know why they're doing them, but it's routine. Maybe even a joke a the lost party member would laugh at, but now there's silence.
So now the party has an EXTREMELY personal hook. Who was this person that was clearly important to them, and how/why did they disappear?
That, to me, is the crux of the false hydra adventure. It's about the impact of the false hydra, not about the monster itself. At least, not yet.
Oftentimes, there is a trick to killing a false hydra. Classically, it's that they sing a song that makes you forget. You can make it anything. Whenever they experience an NPC who lost someone they don't know, maybe there's a scent in the air. Maybe it's a song. Maybe it's a visual or texture or something. The party can figure out the gimmick and protect themselves from it however they want.
I didn't even do that. I just had the only time they actually fought it be the time the false hydra died. There was never a counter to the memory loss except to kill it before it wiped the party's memories of it.
I would also have them 'fight' it before a final confrontation. Not a real fight, but rather a 1 minute time skip. They blink, and there is a hay bale on fire, wreckage around them, they're hurt, lost spells, but no one remembers what happened. Don't play the fight out, just deal damage and tell players what they lost. I tended to do 1/2 health, and 1/4 of their resources, like a regular encounter might cause. This should also have a major aspect of the lost memory thing play up. Maybe it smells strongly of vinegar and petrichor after the 'fight'. The party could protect themselves by plugging their nose or putting herbs under their nose or strongly smelling mint before the fight. Doesn't matter.
In the end, the party should find the body of the lost party member (with loot relevant to their class) and the other victims it erased from memory.
Bonus: Put mild memory-altering effects in the fight, like the repositioning "you thought you were here" kind of thing.
