Ghul-Drâz, hidden citadel

Ghul-Draz, hidden citadel enters the battlefield
tapped. Ghul-Draz doesn`t untap during your
untap step.
If a Zombie enters the battlefield untap
Ghul-Draz. If a Zombie leaves the battlefield,
untap Ghul-Draz.
{t}: Add {b} to your mana pool for each Zombie
you control.