Urth, the Necromancy Artist

Whenever a Zombie you control dies, add {x} {b} mana, where {x} is a number of Zombies in
your graveyard. Spend this mana only to cast
Zombie spells and to use
Urth, the Necromancy Artist abilities.
{b}{b}: Sacrifice a Zombie.
{x}{b}{b}: Target Zombie gains +X/+X and Lifelink until end of turn.
{3}{b}{b}: Search your library for a Zombie card,
reveal it and put it in your hand.