Ritual of Samhain

Ritual of Samhain costs {1} less to cast for each
zombie you control.
When Ritual of Samhain enters the battlefield,
each player sacrifices a creature. You may put
two target nonartifact creatures from any
graveyard onto the battlefield under your control.
They are zombies in addition to their other types.
{b}{b}, Sacrifice three creatures: Return Ritual of Samhain to your hand.