Instant, B (1)
Common
Destroy target creature that was dealt damage this turn. It can't be regenerated.
"What is crueler? To let a wound of the heart fester, or to simply cut it out?"
—Crovax
Illus. George Pratt

4/1/2008: Fatal Blow can’t target a creature that hasn’t been dealt damage this turn.
4/1/2008: Fatal Blow cares whether the creature had been dealt damage during the turn, not whether there is still damage on it. If the creature had regenerated, causing all damage to clear, Fatal Blow can still target and destroy it.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander