Yorick, Shepherd of Souls

Whenever a creature an opponent controls dies, create a tapped Walker token. This ability triggers only once each turn.
At the beginning of your upkeep, if you control four or more Walkers, create The Maiden, a legendary 4/4 blue and black Wraith creature token with deathtouch, lifelink, and "At the beginning of each opponent's end step, the turn player loses life equal to the number of Walkers