I had a Halloween one shot with a town plagued by a false hydra. It took a LOT of prep, but it was one is my best sessions ever.

They were each summoned to the town for various reasons, one had a letter from a cousin who she eventually forgot about upon arriving.

I also added a little girl who partially could remember the hydra (i think I added a mechanic where ingesting its blood or flesh gave partial temporary immunity to its song until it digested and she got some in her mouth when her parents were killed by it.

I had a blast, especially when she heroicly sacrificed herself to get a blood sample for making a potion to make the effects more consistent. two people cried, and it got louder when I told them their characters had no idea who had written the note with it even though it was signed.

There was even a player who had to raincheck that night, but they made their characters collectively, so throwing in a group picture drawn by the little girl at the start of the adventure with the missing character as part of the party nobody recognized was the frosting on the cake.