Vengeful Mystic

Antagonism for Humans (This spell costs {1} less to
cast for each Human your opponents control.)
When Vengeful Mystic enters the battlefield, all creatures get -X/-X until end of turn, where X is
your devotion to black. (Each {b} in the mana costs
of permanents you control counts toward your devotion
to black.)
"Because of them, we couldn't live in unity, so I'll
unite us all in death."