Pod of Necrosis

Whenever a creature you don't control dies, put a charge counter Pod of Necrosis.
{1}{b}, {t}, Sacrifice Pod of Necrosis: All creatures get -X/-X until end of turn, where X is the number of charge counters on Pod of Necrosis. Then all creatures become black Zombies in addition to their other types if 3 or more creatures died this turn.