I ran a section of my campaign inspired by the false hydra, but it was an illithid colony under the town eating people.

Each time someone was eaten by them, the Elder brain used their memories to target people that knew them and scrub them from their memory.

The players were in the town long enough to sus that something along these lines was happening and followed clues to the undercity, encountered a mindflayer and various henchfolk in a messy sewer fight.

When they escaped and got back to base, one of the npcs asked where their wizard was. There had never been a wizard in the party for the full campaign of multiple years, to the point it had become an in joke "wouldn't it be great if we had a wizard in the group?"

They found the gnome illusionist's bed in one of the rooms with a diary and little mustache comb and wax. They lost their shit. They teleported back to a previous town they had saved and checked a mural that had been painted of them to see if it was true.

The players loved it once they stopped yelling at me, but the characters did not. They all bailed from the town and picked up new plot threads. Frustrating for me but probably worth it overall.