Although the Pokémon franchise is usually pretty tame, it can occasionally turn rather dark, with Yamask being a particularly horrifying example of a Ghost-type Pokémon with a dark backstory. Most Pokémon are little more than fantastical animals, and their behavior and lore often innocently reflects that, but every so often there are Pokémon that border on something from a horror game. The most recent example is Legends: Arceus' Basculegion's dark origin story, but Yamask's is even more unsettling and has some very chilling implications for the Pokémon world.
Yamask is a Ghost-type Pokémon initially found in the Unova region with the release of Generation 5's Black and White. Although the Unova region is generally based on New York, Yamask's appears to be influenced by ancient Egyptian motifs. Its large red eye is reminiscent of the Eye of Horus, and the mask it holds in its tail bears a resemblance to ancient Egyptian death masks. The death imagery is nothing short of intentional, because the lore surrounding Yamask's existence is surprisingly gruesome.
Yamask is generally found in ruins, "carrying a mask that's said to have been the face it had when it as still human," according to the official Pokémon Pokédex entry. Yamask isn't so much a zoological creature as it is a human spirit, somehow trapped in its current form. The poor thing apparently retains its human memories, desperately clinging to its former visage, even reportedly shedding tears while looking at it. A Yamask will become incredibly upset if separated from its mask, and possesses any human who dares to wear it, making Yamask a Pokémon capable of human speech. Its preference for ancient ruins seems to imply that some Yamask lost their humanity long ago.
Galarian Yamask Is Just As Horrifying
When Pokémon introduced Galarian forms with Gen 8, Yamask was amongst the Pokémon to get a makeover. Instead of holding a mask, Galarian Yamask's tail appears to be trapped in a clay slab. According to the Pokédex, the slab with cursed engravings "is said to be absorbing the Yamask's dark power." This implies the Galarian Yamask is trapped in some sort of torturous state if its mask is nowhere to be found. Eventually the Galarian Yamask's human spirit is entirely absorbed by the slab and it evolves into Runerigus, a cursed stone painting.
The fate of a Galarian Yamask Pokémon is actually much worse than is original, Unovan counterpart. A regular Yamask will evolve into a Cofagrigus, whose sarcophagus shape also evokes the ancient Egyptian death motifs, and will no longer remember its human origin. There isn't any mention of a curse in Cofagrigus' Pokédex entry, so hopefully it has a more peaceful evolution into a full-fledged Pokémon. The implication that humans can transform into Ghost-type Pokémon like Yamask, and uniformly long to return to their previous form, is extremely unsettling for a normally lighthearted series like Pokémon.
Source: Pokémon