After obtaining it, turn back and head left toward the stairs. You can pick up another Dust item ahead, then continue up the stairs on the left.
Once you come up, you'll reach a save point. There's also an NPC on the right you can talk to.
There is a Dust item on the roof at the far left of the save point area.
Next, go talk to that person. After speaking with him, you'll receive the quest "Isolation Wedge," where you need to help him find the "Hunyuan Stone." This Hunyuan Stone is actually on the first floor, but since we don't have the key item yet, we can't retrieve it for now.
