Balilin

Balilin is a Legendary Pokémon that appeared one day to save the world from Eternatus striking back in an even stronger form knows as Ultimax it must evolve to be able to enter its Hero Forme and ultimately get strong enough however.

Evolutionary line:

Balilin: save 15 corrupted Pokémon (swapped to level 40 in games without corrupted Pokémon) --> Balimanin: when Eternamax Cannon learned --> - Balultimate