Ominous Cube

{t}: one land you control gains ({t}: add {u}{u}{u}{b}{b}{b} to your mana pool) in additional to its normal abilities
{t}, exile all non-land permanents on your battlefield: every land you control gains ({t}: add {w}{u}{b}{r}{g} to your mana pool) in additional to its normal abilities
Ominous Cube cannot be exiled by any spell or ability