Callith, Headstalker

Menace

When Callith, Headstalker enters, if you paid its dash cost, Callith, Headstalker loses menace and you sacrifice half the creatures you control, rounded up.
If you do, the next time Callith, Headstalker deals combat damage to a player, they lose the game.
If you didn't play its dash cost, choose target creature. Its controller sacrifices it.

Dash {4}{b}{b/r}