Surrender to Beasthood

Each creature you control without monstrosity has "{x}{b}{g}: Monstrosity X."
(If that creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)

Whenever a creature you control becomes monstrous, it fights target creature you don't control. That creature's controller loses 1 life
and you gain 1 life.