Yorn-el Tar, First General

{2}{u}{b}: Corrupt 4. (If you cast this spell for it's
Corrupt cost, put it under target creature your
opponent controls. At each of that player's end
steps, put a corruption counter on that creature.
Remove 4 corruption counters and destroy
corrupted creature. Then return your creature
to the battlefield with all the abilities of the
corrupted creature.)

Demon creatures you control have Swampwalk