Sorcery, X1B (2) Spend only black mana on X. Drain Life deals X damage to target creature or player. You gain life equal to the damage dealt, but not more life than the player's life total before Drain Life dealt damage or the creature's toughness.1
Illus. Richard Kane Ferguson
You can put as much black mana as you want into this spell's X value, but you cannot gain more life than the creature's toughness. [D'Angelo 1996/07/01]
You may gain up to the total toughness of the creature even if it was already damaged. [PPG Page 70]
Cost reducers, such as Helm of Awakening can be used to reduce the X part of the mana cost. [WotC Rules Team 1997/06/01]
Note - Before errata, the X part of the cost was not part of the mana cost; it was part of the card's text. [Oracle 1999/07/01] This means the X cost is now part of the cost for things like Spell Blast.
Note - The artist's name, Shuler, was misspelled on the Limited Edition and Unlimited Edition versions of this card.