Quote Originally Posted by Kaertos
Does Xykon start at the top of the battle stack when he appears in the game or does he move to the back?
Xykon should be the first monster you face in the room where he appears. This ensures that he gains all of the support benefits from the other monsters that are played into the room.

Note: This is true for all Xykon Monster cards (not just the Xykon card). As the first card played into the room, the Xykon Monster card always goes at the top of the battle stack and is therefore encountered first in battle.

The only thing different about battling Xykon is that you never actually get to fight the other monsters in that room. When Xykon is defeated, the dungeon begins to collapse and the other monsters in the room immediately flee the dungeon.