The Green Door

Whenever a creature you control dies, put a Gamma counter onto The Green Door. During your precombat main phase, add X mana of any combination of colors where X is the number of Gamma counters on this card.

Creatures you control have "Mill a card: Regenerate".
"Before him stood the Green Door, a gateway to endless rage and resurrection - a cycle he could never escape, yet always faced with unyielding resolve."