Corrupted Countenance

Whenever you gain life, choose that many
creatures controlled by your opponents. As long
as those creatures remain under an opponent's
control, they get -1/-1.
If Corrupted Countenance would cause you to
kill a creature, instead gain control of that creature.
It becomes a black demon in addition to it's other
types.
If Corrupted Countenance has been triggered ten
or more times and you control five or more
demons, you win the game.