Kleth, Bringer of Pain

As an additional cost to cast this spell,
you may sacrifice any number of black creatures,
This spell cost {2} less to cast for each
creature sacrificed this way.
When Kleth, Bringer of Pain enters or
leaves the battlefield, each opponent loses
X life where X is the number of {r}and {w}
mana in your mana pool and you gain that
much life.
Sweet, sweet pain