Yamask evolves 2 ways

Stages

Yamask starts a line of two stages, from Generation V

1. Yamask#0562ghost
2. Yamask to CofagrigusLevel 34

   Yamask to RunerigusTake damage, then walk under the arch, at Dusty Bowl, after taking 49 damage, as the Galar form
   Cofagrigus#0563ghostRunerigus#0867groundghost

How each step works

2 steps in the Yamask line, the same in every game that has them