Altar of the Necromancer

At the beginning of your end step, remove 1 sacrifice counter from Altar of the Necromancer
if able. Then if Altar of the Necromancer has 0 sacrifice counters on it, you lose the game.
Sacrifice a creature: Put a sacrifice counter on
Altar of the Necromancer.
Remove a sacrifice counter from Altar of the Necromancer: Return target creature from your graveyard to the batttlefield.