Okay, to start with, I LOVE the idea! I agree that this would be a good counterpart to angel and archangel. I feel that the ability is a bit OP. The angel can only heal creatures up to 5 HP, which although very useful in the right situation, is very situational in that very few decks are built around having angels in play to heal severe damage. This card not only lowers HP, but also attack, and gives a buff to death and darkness creatures, which is an ability versatile enough to build a deck around. Perhaps if the ability cost 2
it would be more reasonable, since the damage dealt to opposing creatures (non-death and non-darkness) is not able to be healed at all and due to the weakening, and also for the buff to death or darkness creatures. To justify the cost of 2
, think of this situation:
-Blessing costs 3
and gives target creature +3/+3
-Corrupt (this card's ability) costs 2
(in this scenario) and gives target Dark creature (which would probably be an ally pre-selected to be in this deck partly because of the ability of Fallen Angel) +2/+1, +4/+2 over 2 uses.
You also have the fact that darkness has the vampires in its artillery, which, when combined with this card's buffing effects, could potentially deal massive damage and heal you very much every single turn. A fairly common combination in current use is the Blessings/Vampire deck, which uses two elements, and is therefore much more balanced than the massive buffing and vampire power that an all-darkness deck with this card in its line-up alongside vampires would be. All in all, I think that the raise in ability cost to 2
would be beneficial balancing-wise, but I'm not sure that even a raise in cost would balance out this card. Once again, I love the idea, but I do think that it needs some balancing to be a really great card. Keep up the good work!