I wasn't able to get the custom creation kit so I had to make due with Photoshop... Enjoy!
When I think of white dragons, THIS is what pops into my mind first! Tweaked card art from an upcoming boss in BM2! (Yu-Gi-Oh Reference) [Spritework Credit: Ravenm]
The strongest Magician in terms of Attack and Defense. After making the Blue Eyes I just couldn't resist making it's ultimate rival... I just HAD to! (Yu-Gi-Oh Reference) [Spritework Credit: Spritesliker007]
Cursed Shrine gains +2 if your boss also has a Cleric treasure type. (This includes Kirax after it Levels Up)
Wicked Shrine Version 2: Glowing - Discard up to 3 spell cards. Until end of turn, this room gains +2 for each discarded card.
The Ice King from Adventure Time. Grants Immunity from Freeze to all rooms in your dungeon if you have at least 3 wounds. Opponent's must think up other ways to stop you!