So following up, I tried the method of my Yamask taking 49+ damage in Wispy Path, yet it still refuses to evolve. I feel like I am missing something...
So i've searched quite a bit and couldn't find the exact way to evolve Yamask into Runerigus in Rejuv. I saw a Reddit thread mentioning it evolves on the Wispy Path, but I could not figure out where exactly. Has anyone done this?