In Pokémon Legends: Z-A, Galarian Yamask evolves into Runerigus only after two very specific things happen. First, it has to take some real damage; then you’ll need to stand under one of the bridges at Coulant Waterway in Lumiose City. The trickiest part might actually be catching one: Galarian Yamask appears in Hyperspace Lumiose and its spawns are RNG-dependent, so patience helps.
Galarian Yamask arrived via the Mega Dimension DLC and lives in the Hyperspace Pokédex. Don’t have the DLC? You can still use it in the base game if another trainer trades it to you.
Here’s how to evolve Galarian Yamask into Runerigus in Pokémon Legends: Z-A without missing a step.
How to evolve Galarian Yamask into Runerigus in Pokémon Legends: Z-A
Galarian Yamask will evolve once both conditions below are met:
- It has taken at least 49 points of damage (it must still be conscious).
- You are standing under one of the two bridges at Coulant Waterway.
The damage requirement can be easy or dicey depending on your Yamask’s level and HP. A safe approach is to enter a low-level wild area and let weak Pokémon chip it down until it’s taken 49+ total damage without fainting. Avoid recoil and status effects that might tip it over the edge.
With the damage done, head to Coulant Waterway on the east side of Lumiose City. On the map, it sits between wild zone 2 and wild zone 17, and you’ll see two bridges spanning the water.
Coulant Waterway sits on Lumiose City’s east side, with two bridges crossing the channel. 
Under either bridge, the evolution icon appears once Yamask has taken 49+ damage.
Stand beneath either bridge with your damaged Galarian Yamask in the party, open the menu, and you should see the evolution arrow appear. Confirm the prompt and it’ll become Runerigus on the spot.