Use
Nightborne Glow creates a firefly-light effect and is used at the end of Flickering Hope / A Radiant Resolve around Goldflow Slope.
How to Obtain
If the skill is not yet owned, open its entry in the current Mystic Skill menu and use the Tips/Clue route. Hidden Mountain quest requirements can change between hotfixes, so the live client is the source of truth for its acquisition trigger.