Katarina, The Sinister Blade

First Strike, Flash, Haste
{1}{r}{t}: blocking creatures your opponents control gain -1/-1 until end of turn. If a creature dies this way, you may search your library or graveyard for a card named Blur of Blades, Exile it, then copy it. you may play the copy without paying its mana cost. You may copy the spell whenever another creature your opponent controls dies this way until end of turn. Remove any counters added by Blur of Blades at the beginning of the end step.