Right then. This question is about the "I forgot they could do that" Screw This! card, and Xykon himself.

We just played a game in which a player used this card to get rid of Xykons, "Xykon" special ability. We were unsure how to rule this.
We played it as the dungeon did not collapse when he was defeated because it was his special ability that triggered it. Which, made the endgame very weird.

How should this be ruled?