In Honkai: Star Rail, you will have to complete various quests to progress in the game, and one of them is the Fleeting Lights quest you need to complete to unlock Forgotten Hall, which is the Spiral Abyss in Genshin Impact. However, you won't find this mission in your quests section, and if you are confused about how you can unlock and complete the Fleeting Lights quest in Honkai Star Rail, here is a detailed guide for you.
Honkai: Star Rail Fleeting Lights Quest Guide
You will get this quest from Pom-Pom after you hit Trailblaze Level 23 in the game, and if you don't know how to increase your Trailblaze Level, you can follow our dedicated guide for it. You will suddenly receive a message from Pom-Pom that you can check in the Messages section by opening the Main Menu of the game, and after having a small conversation, you will accept the Fleeting Lights quest.
Go to the quest section, click on Navigate button and then go to the location marked on the map below in Parlor Car by teleporting there.
Once you are there, talk to Pom-Pom, and they will ask you to investigate the strange occurrence at the Express; however, no exact location will be given to you. To find the strange occurrence, go towards the stairs in the quest area, but don't go up. Instead, take a right, and you will see a broken mirror with a portal next to a plant.
Investigate it, and a Messenger will appear in front of you. Complete the dialogue with her, and Forgotten Hall will get unlocked for you. Interact with the Messenger again to enter the Forgotten Hall and clear its Memory Stage 1.
After you clear it, another dialogue with Messenger will initiate that you will have to complete and then go to Welt Yang in the Express. Talk to him, Pom-Pom, and Himeko, and once the conversation is over, the Fleeting Lights quest will be completed. You can now come back anytime to play Forgotten Hall.