Don't forget about the part where it drains all your . Is that not enough?
Black quanta is not hard to come by especially with the use of Devourers/Pests. The same can be said with Light (miracle) but devourers are far more useful than RoL in most decks. (ALSO... Miracle is more situational and cannot be spammed/played at any moment. Corruption is not situational and rather devistating)
The problem with the card is how powerful it can be. It would demolish decks that relied on dragons/ffq/nymphs and pharohs. Why even consider running dragons in your own deck if you can steal the opponents win condition? (cards that win them the games). Most people don't run 6+ cards that make them win, they usually carry a few dragons (2-3) and a few meta-game creatures. Running 6 corruptions would demolish most decks and even running 2-3 corruptions would be game breaking/overpowered. A cost increase does help balance the card but it does not change how overpowered the card is..
Suggestions:A. Cost increase and Dragons/Nymphs/Creatures over 7 hp cannot be targeted.Reasons: Dragons/Nymphs are 'imbued' beings. They are special in the sense. (It would also be overpowering to steal them). While creatures over 7+hp overcome the will of the 'shadow' and cannot be corrupted.
B. ~Same cost. Paralyze and create a 'shadow clone' of target creature. Length of Paralzsis/ShadowClone is determined by creatures current HP. Max duration 5 turns and 1 turn minimum. (Creatures with cost abilities get -1 from their duration)Reasons: Would not demolish the opponent. The idea is: stronger the creature the more resistant to corruption.
C. Keep current card and add another effect--You become poisoned. Poison strength is equivalent to creatures total stats divided by two--rounded down. (The stolen creature loses all status effects. Poison, freeze, time bubble)Reasons: The stronger the creature, the more backlash you receive. Although stealing strong creatures is devastating to the opponent there is a price to pay... It also gives more purpose/application to other cards. Such as: Parasite, Plague and Purify.