Fleeting Lights is an Adventure Quest in Honkai: Star Rail. This is a simple but very important quest to complete as it gives you access to the Forgotten Hall, a place where you can have a rematch with stronger versions of previous bosses to get tons of rewards and useful resources. This is, alongside the Simulated Universe, the main end-game content in Star Rail, so you might wanna start getting into it as soon as possible.
How to Complete the Fleeting Lights Mission and Unlock the Forgotten Hall
Fleeting Lights will appear as an Adventure Mission triggered after Pom-Pom sends you a message asking you to visit the Express, and will then be available in your Missions log. The message should pop up after reaching Trailblaze Level 21. Teleport to the Parlor Car and speak with Pom-Pom. The small bunny is worried about the presence of something that he cannot see but can definitely feel in the Express.
It’s up to you to solve this problem, so take your time examining all of those other completely unrelated things until you decide to go for the mirror that just appeared out of nowhere. A female figure jumps out of it and will start explaining about the Forgotten Hall and how the Garden of Recollection is interested in your memories. The usual protagonist rizz.