Fatal Blow

Oracle Text

Destroy target creature that was dealt damage this turn. It can't be regenerated.

Card Rulings

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.