Festerbound Colossus

At the beginning of each combat, you may put a
rot counter on target creature. If you do, that creature is a black Zombie in addition to its
other colors and types and has decayed for as
long as it has a rot counter on it. (It can't block.
When it attacks, sacrifice it at end of combat.)
If a creature with decayed would leave the battlefield, exile it instead of putting it anywhere
else.