Basculegion official art

Notes:

From the official site:
Water/Ghost
Basculin in the Hisui region can evolve into this Pokémon. This Evolution occurs when a Basculin is possessed by the souls of other Basculin from its school that could not withstand the harsh journey upstream. Basculegion fights together with these souls, which attack opponents as if with a will of their own.

The moment this Pokémon senses animosity, it will become enraged and attack relentlessly until the enemy is defeated. This Pokémon gains power from the souls possessing it, letting it swim on and on without tiring.