Decade of the Still-Moon

Whenever a Creature would be sent to the Graveyard from the Battlefield, instead Tap it and put a Death Counter on it.

If a creature has 5 or more Death Counters on it, it loses all abilities and becomes a 1/1 Zombie in addition to its other types with “This creature cannot be Sacrificed.”

Whenever a Creature enters the Battlefield, its Controller Sacrifices a Creature, then Exiles any one Creature with a Death Counter.