The God-Devouring Serpent

Protection from legendary
Whenever you sacrifice a creature, exile it with a tribute counter on it. Then, if there are three or
more cards in exile with tribute counters on
them, transform The God-Devouring Serpent.
{b}, Sacrifice a creature: Put a +1/+1
counter on The God-Devouring Serpent.
Regenerate it.