Exava's Cauldron of Blood

{t}: Add {1} to your mana pool.
{t}: Add {r} to your mana pool. When that mana
is spent to cast a red or black creature spell, that creature enters the battlefield with unleash. You lose two life. (You may have that creature enter the battlefield with a +1/+1 counter on it. It can't block
as long as it has a +1/+1 counter on it.)
"Feast on the lifeblood! It shall stir up your hatred!"
-- Exava