Keeper of the Dead

Oracle Text

B, T: Choose target opponent who had at least two fewer creature cards in their graveyard than you did as you activated this ability. Destroy target nonblack creature that player controls.

Card Rulings

10/1/2009 A different opposing player may be targeted each time the ability is activated.
10/1/2009 It is only necessary that the condition be true as you activate the ability. The ability will still resolve normally even if the condition is no longer true at that time.