Creeping Decrepitude

This spell costs {1} less to cast for every two
untapped black creatures you tap while casting it.
{3}{p/b}{p/b}{t}: If you control 10 or more untapped
black creatures, gain control of all non-
planeswalker, non-legendary permanents target opponent controls. Those permanents lose any of
their colors and are now black.
Total colonization was always its final purpose.