"Nights of Wundagore" is a 3-part story arc featured in the Avengers Vol 1 #185-187, which was written by Mark Gruenwald, Steven Grant, and David Michelinie.
Synopsis[]
The Elder God Chthon made the sorcerer Modred his thrall and had him capture the Scarlet Witch as part of his plan to use her as his vessel on Earth.