Stone Tower, Curse of Ikana

{t}: add {w/b}
{t}, Sacrifice a creature: Search your library for a
black creature and place it in your hand.
This card enters the battlefield tapped.
All creatures you control are only black mana
color, if this card leaves the battlefield, all
creatures under your control are now white as
well as their other colors.
If you have an Army with 8 or more counters,
you may pay {b}: Exile this card and destroy target permanent.