Bygone Times Like Dust Passing

First things first, head back to where the Jade Chamber usually hangs out, just outside The Golden House. You'll need to chat with Beidou and Yun Jin. After that, make your way back into Liyue Harbor and find Blacksmith Master Zhang. Your goal is to get some info on these 'Wonder Cores,' but Zhang's got a condition: you need to bring him some Subrosium and Starsplinter Iron first.

Once you've got the quest from Zhang, it's time to hunt for that Starsplinter Iron. Use the teleport waypoint southwest of Mt. Tianheng. As soon as the scene wraps up, walk just a few meters behind you. You'll trigger another scene, and then you're on the hunt for two pieces of Starsplinter Iron. Climb up the nearby rock formation to snag the first piece. For the second, just look straight ahead from where you are – it's sitting right there.

With the Starsplinter Iron in hand, you need to find some Subrosium. Head over to the village located southwest of your current spot. Your mission here is to talk to a villager who might know where to find it.

After chatting with the villager, you'll need to search for some old records. Look for a golden marker near a destroyed wooden shack, just up the wall to the left of the villager you spoke with. Interact with it, then head into the abandoned building and find another marker near some barrels. This will give you some backstory about Shenhe's father. Keep going upstairs and enter another broken building; you'll find the final marker here, which will point you directly towards the Subrosium.

Now that you know where to look, return to Shenhe. She's hanging out near the villager you spoke to earlier. After a quick chat, head over to the rock in the middle of the lake. Here's a crucial step: you need to forward the time to 17:30. This will trigger a short scene. Once that's done, walk over to the west side of the lake. You'll spot the Subrosium sitting there, conveniently located opposite Xiao.

Finally, with both materials collected, return to Master Zhang in Liyue Harbor. A scene will play out, and this quest will be completed!