Olethros, The Calamitous

When Olethros enters the battlefield, put a
number of 0/0 green hydra tokens with a +1/+1 counter on them and that cant block equal to the
amount of mana spent to cast this spell.
Sacrifice a hydra: Put a +1/+1 counter on target
hydra
Whenever a hydra leaves play, create twice that
many hydra tokens that cant block at the
beginning of the next end step.