Spend only black mana on X.
Drain Life deals X damage to any target. You gain life equal to the damage dealt, but not more life than the player's life total before the damage was dealt, the planeswalker's loyalty before the damage was dealt, or the creature's toughness.
Cost reducers can be used to reduce the X part of the mana cost.
You may gain up to the total toughness of the creature even if it was already damaged.