Now this be my worst nightmare. Chosen One + Stall. Essentially, this prevents you from discarding 12 cards, if you have 41 or more cards. This combo is in my book as OP, and I intend to prove it, after I achieve my Nihility form (Find a weakness, cyborg, and mage hunter).
By reducing the amounts of cards they can potentially lose to near 0, it becomes harder and harder to get rid of them. Eventually, no matter how hard you try, they are simply too resilient to be removed from the game.
Most of their other skills are balanced enough, but since Champion of Light locks down around half the creatures one can have... I'd love to add that to my Nihility form.