Yamask 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. Yamask is capable of speech when possessing a human, but not in its regular form.
Due to being a human long ago, Yamask retains its memories from before it became a Pokémon. It becomes furious when the mask is lost or taken away. If someone wears the mask Yamask carries, they will become possessed by the Yamask in question. Sometimes, it looks at this mask and cries, apparently in longing. Yamask is known to wander the ruins of ancient civilizations.
Galarian Yamask, in contrast to its Unova variant, has purple eyes and has an ancient clay tablet directly attached around its tail. Galarian Yamask is said to have been born when that tablet was drawn to a vengeful spirit. The tablet bears cursed engravings that resemble a serpent's tail and is said to absorb Yamask's dark power.
Yamask debuted in A Night in the Nacrene City Museum!, where it was causing havoc in the Nacrene City Museum. This eventually turned out to be because Yamask's mask had been put on display after it accidentally dropped it while sleeping. In the end, Lenora, along with Ash, Iris, Cilan, and Hawes gained an understanding with it and gave its mask back. Yamask then left the museum peacefully.
Yamask may be based on an Egyptian ba, a person's soul, typically depicted as a bird with a human head, or šwt, the shadow or silhouette of a person's soul - both appearing after the person's death. Its overall body shape is similar to the Eye of Horus. The mask on its tail is likely a death mask.
Galarian Yamask holds a clay tablet that may be based on Ingvar runestones instead. It may also be based on a Deildegast, a Norwegian folklore spirit that is forced to haul around a boundary stone as punishment for moving one while it was alive. The fact that Galarian Yamask evolves into Runerigus after taking at least 49 damage could be a very subtle reference to the Bardo Thödol, a Buddhist concept of Purgatory after death and before reincarnation; this process of Buddhist Purgatory is said to last 49 days.
Yamask may be a combination of 闇 yami (darkness), や ya (bad), or Yama (lord of the dead in Hindu and Buddhist mythology), and mask.
Desumasu may be a combination of death and mask. It is also a pun off of ですます体 desumasu-tai, a form of polite Japanese which gets its name from the fact that sentences often end in です desu or ます masu.