That's Halloween content...just like the Doom event and the Kotl City drops.
The Sorc Dungeon is a bit different...one room spawns at a time...so when you go in, it will be empty except for one room...clear the spawn in that room and kill the mini boss to make the next room activate. Once you have completed all room, Jack the Pumpkin King will spawn and then you kill him! It's just a fun way to get drops to turn in for the new items.