Exponential Swarm

{2}{b}{b}: Create X 1/1 Bat creature tokens
with flying and "{2}, {t}, Sacrifice this
creature: This creature's controller gains
2 life. Any player may activate this ability.", where X is the number of bats you control.
The sky darkened, the moon became black, and a great thunder rippled throughout Innistrad. Bats swarmed the skies, with Arahta at the head, and none stood in their way.