When they battle, pokemon are wounded, but rarely die; typically, they go to a pokemon center to heal up, and treat the fact of injury as part of the game. Yamask, however, are poor at coping with injuries, and while they may be best known for their painful memories, their reaction to physical wounds is little better. (Admittedly, this distinction is not very clear when dealing with ghost pokemon). When a sufficiently injured Yamask crosses under the site of the Battle of Dusty Bridge – perhaps the deadliest conflict of Galar's civil wars, when a packed bridge collapsed under arrow and Cinderace fire and thousands plunged to their deaths – a Runerigus is born.
The desire to avoid the pain of defeat by getting stronger drives many a pokemon evolution, but there is something different about a Runerigus' spirit. They do not fight so much as take revenge, and consider their opponents' minds fair game not just for trash talk or disruption, but to transmit the agony of their own memories. It is not unheard of for pokemon (or in a couple famous incidents, even trainers) to retire after a Runerigus encounter – which is rendered all the stranger by that, in most of said battles, the Runerigus lost!
It is possible, by learning an ancient alphabet and unscrambling its markings, to learn the cause of a Runerigus' agony. Not the truism that pokemon battles hurt, but whatever pain they faced, so long ago (for no Yamask has been born in Galar in a thousand years) that made their spirit linger here to begin with. One need only face a Runerigus and read it aloud to allow its spirit to at last move on.