Desumasu

Desumasu is a shadow-like Pokémon holding a golden mask that has a face from its time as a human. It has two shadowy, tendril-like arms and large, red eyes. It also has a small tail-like appendage that holds the mask. Desumasu is capable of speech when possessing a human, but not in its regular form.

Due to being a human long ago, it has memories of being a human. It becomes furious when the mask is lost or taken away. If someone wears the mask Desumasu carries, they will become possessed by the Desumasu in question. Sometimes, it looks at this mask and cries, apparently in longing. Desumasu is known to wander the ruins of ancient civilizations.

Galarian Desumasu basically looks like its Unova variant, but it has purple eyes and a small piece of an ancient clay tablet is directly attached to its tail. The tablet bears cursed engravings and is said to absorb Desumasu's dark power.

(Source: Bulbapedia)