Icefractal has the link to the post where the False Hydra originates. For those curious who don't want to follow the link, it basically is a monster that consumes people and erases the memories of those who knew them, via a song that also hides it from perception. It makes for an interesting psychological horror adventure if the DM puts in the work and the players are looking for such. It is definitely meant as at least an adventure arc, rather than a singular encounter.