Gate to the Underworld

Whenever a creature dies, put a key counter on
Gate to the Underworld.
At the beginning of your upkeep, each player sacrifices a creature.
Remove 7 key counters from Gate to the
Underworld and sacrifice Gate to the
Underworld: Put all creatures from all graveyards
to the battlefield under your control. Each
opponent discards half of their hands, rounded
up.