Ambitious Grafcaller

Whenever another nontoken, non-Zombie creature you control dies, put a charge counter on this creature and a 2/2 black Zombie creature token onto the battlefield. Then, if you control four or more Zombies, sacrifice this creature and return it to the battlefield transformed with an additional charge counter on it.
"Another failure tonight. Though my mastery of the Art grows ever greater, its greatest mysteries remain beyond my reach."