I think the perfect set up for a false hydra is actually a party of newer RPG players.
For the unfamiliar: A False Hydra is a monster with a very particular gimmick, namely that it makes itself, and its victims, be forgotten using an enchanting song.. It's essentially 'The DM gets to gaslight the players a bit, as a treat' the monster.
Usually, the setup is the players arrive in a town where things seem peaceful, if quiet, and steadily realize that people that should, by all rights exist, simply aren't there. The innkeeper has a child, but claims she doesn't even know the man in the family portrait. If you press the matter she gets upset. You eat fresh baked bread for dinner, but in the morning, there has simply 'never been' a baker. That kind of thing.
And of course, the classic of 'You wake up. There's an extra bedroll. you don't know why, but the fact that it's empty fills you with dread.'
But of course, implying you had another party member who just got eaten and forgotten kinda fucks over the story you've been telling this whole time. Shouldn't all those fights have gone differently? How are the players going to care about a character who literally exists only as a supposed hole in their memories?
That's why, for this to work, I recommend new players (or just act like you're running a slightly laxer table).
Essentially, figure out what basic utilitarian weaknesses the party has, and then, without any great fanfare, handwave them. Suppose they don't have a wizard. They find a magic item. You say, 'yeah, you get it identified, and it's a ...' Without particularly specifying how or with whom. They're in a dark cave, and the one human of the group is like 'hey, can I even see?' and you tell them, 'oh, yeah, don't worry about it, just pretend you had Darkvision cast on you.'
If they press the issue, you make up a joke, not-actually-real character, who you say is actually doing all this stuff. Blinzki the Narratively Convenient Wizard or whatever. Just a narrative construct, there to grease the wheels of gameplay. (Though, keep score a bit. Remember where things by all rights shouldn't have worked)
And then, they'll get to this town, and you'll feed them a potentially magical item, and they'll ask what it does, and if you're very, very lucky, you'll get an exchange to the tune of
'Do you know anybody who could identify it?'
'We've always been able to identify things before.'
'Yeah. You think about that, and shiver with a sense of wrongness.'
You find the corpse of a wizard. Her journal references people with your names, and she talks about how lucky she's felt to be traveling with you. She's not a great combatant, usually hiding when battle starts, but she's made herself useful with in other ways, casting darkvision on the fighter when they're in a dark cave, and enhancing abilities at opportune moments.
You don't know her.
You're crying at the sight of her corpse.










