Go to the ruins near Guili Plains shown on the map above. You will find some rocks with a barrel bomb close to it. Detonate it, and there will be a ruin guard guarding the basement's entrance.
Once you take out the ruin guard, you can go to the basement and pick up some treasures. Read the worn letter there to start the quest.
Bring a character that can use Pyro and go to the quest destination. Be prepared to fight some enemies because once you light the fire, you will get ambushed.
Go to the highest point in Cuijue Slope, which is not that far from the teleport waypoint. If your stamina is not that high yet, you might want to prepare some stamina food.
Once you reach it, get close to the tree and there will be an option for you to "dig" and get the treasure.
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