The Millennial Mountains is a World Quest in The Chasm surface, Liyue.
To start this quest, go to The Chasm’s Maw teleport waypoint then head east to the marked area. In this area, you will met Wang. Talk to him and he will ask you to gather 6 items for offering to Millelith memorial.
When Wang explaining every item, cutscene will zoom in to 6 locations where you can find said item:
Table of Contents
Cup of Commons
You can get Cup of Commons from Precious Chest in the end of world quest: Undetected Infiltration (Yanbo quest).
Flower of Farsight
Located in The Cinnabar Cliff. From Statue of The Seven climb the mountain with Anemo mechanism to boost your jumping. In there you will met Tang Wuchou and nearby you will see 2 Qingxin and 1 Flower of Farsight.
Helm of Warding
Go to wooden house in The Chasm Maw. Follow Seelie outside this house, down to a hole, then enter the next room. Burn the hay with exploding barrels nearby or use pyro, then investigate to obtain the Helm of Warding.
You can find Skyfeather in a cliff above teleport waypoint near The Chasm entrance. It’s a cliff with wooden pillar, bird nest, and 2 Qingxin.
Sundial of Ages
Northeast of The Surface teleport waypoint, you will find table set in front of treasure hoarder and some crystalflies near it. Investigate the bowl and you will get Sundial of Ages.
Go to the marked location, mountain southeast of The Chasm Maw teleport waypoint. In the top you will find investigation points of Warrior Spear and two sweet flowers.
After you get all 6 items, go back Wang and offer all items to memorial.
Hole in the nearby rockpile will opened up and then you can enter to claim your treasure. 1x Luxurious Chest and whole set of 4 star Tenacity of Millelith artifact.
After that, you can talk to Wang to end this quest and obtain your next reward.