I once ran a Christmas-themed Call of Cthulhu one-shot in which a servant of Yog-Sothoth performed a ritual to turn Santa Claus into an avatar of his god, based on their metaphysical similarities re: space-warping abilities.
This led to kids staying up late to see Santa coming down the chimney, beholding something eldritch and suffering massive SAN loss. The Arkham Asylum ended up needing a special children's ward, which was funded by a local philanthropist named Charles Dexter.
I think maybe one of my players might have forgiven me for it by this point for sending them to the Charles Dexter Ward.
