To Thee My Tender Grief Confide

Talk to Nantuck: First things first, you need to find Nantuck, the head of the Mondstadt Fishing Association. He's chilling on the coast, just east of Mondstadt. So, teleport to the central waypoint in Mondstadt, head out the northeastern exit, and you'll find him by the sea. Chat with him; he'll tell you about Jiawei's info and mention a book he wrote about the Lunar Leviathan, which apparently is a giant sea turtle! He'll point you towards Wyrmrest Valley in Dragonspine for fossil evidence.

Head to Dragonspine: Make your way to Dragonspine using the Wyrmrest Valley waypoint. Follow the path north until you reach the objective marker. You'll find a camp with a few ore piles scattered around.

Investigate the Ore Piles: You'll see three ore piles. Two are on your left, behind a torch, and the third is on your right, also behind a torch. Interact with all of them. Paimon will notice they're just ore scraps, but it looks like something was taken from them. Just as she says this, she'll spot a Treasure Hoarder nearby and suggest following them.

Tail the Treasure Hoarders: This is a stealth section! You'll see an eye icon above the Treasure Hoarder's head. Get too close, and the icon fills up, failing the quest. Stay too far, and you'll lose them. The best way to handle this is to wait by the first torch until the Treasure Hoarder gets near the second torch. Then, casually walk towards the second torch and wait again. It's not too long, maybe about ten seconds. You'll overhear the Treasure Hoarder, Old-School Qi, bragging about a gem he found. His buddies, Fleein' Lin and Klutzy Bazi, will dismiss it as just a rock. Right after that, they'll spot you, and you'll have to fight!

Defeat the Treasure Hoarders: You'll face a Treasure Hoarder Hydro Potioneer, a Treasure Hoarder Handyman, and a Treasure Hoarder Scout. It's a pretty standard fight, but it's a good idea to take out the Hydro Potioneer first since they can be a bit annoying. Once they're dealt with, you'll get the 'Fossil Bag.' Paimon will comment that the fossils do look a bit like turtle shells and suggest taking them to Kujirai Momiji.

Talk to Kujirai Momiji: Head back to Kujirai Momiji at the Inazuma Fishing Association, which is northwest of Inazuma City. After a scene, she'll ask about Jiawei and Nantuck. You'll explain Nantuck's book title, 'How The Lunar Leviathan Is Actually A Giant Sea Turtle.' She'll be pretty shocked and mention her dad went looking for the Leviathan and never returned. She's skeptical but says she'll grab a copy of the book. She'll then tell you she has more training for you and to wait for good news. This wraps up the quest and unlocks the next challenges!