Gluttony's Abyssal Maw

Gluttony's Abyssal Maw enters the battlefield tapped.
{t}: Add {b} or {g}.
{b/g}{b/g}, {t}, Sacrifice a creature: Add an amount of {B} or {G} equal to the sacrificed creature's power. Use this mana only to cast creature spells or activate abilities of creatures. You gain life equal to the sacrificed creature's toughness.