Soul Burn

Spend only black and/or red mana on {x}.
Soul Burn deals {x} damage to any target. You
gain life equal to the damage dealt, but not
more than the amount of {b} spent on {x}, the
player’s life total before the damage was dealt,
the planeswalker’s loyalty before the damage
was dealt, or the creature’s toughness.
"Some things are just easy to set on fire."