Kayn, the Shadow Reaper

First Strike
Whenever a creature dies, put a charge counter on Kayn. Then if Kayn has 5 or more, choose one:
-Kayn becomes a 4/6 Demon Assassin with Lifelink, Trample, and First Strike.
-Kayn becomes a 6/4 Spirit Assassin with Deathtouch, Menace, and First Strike