Quick answer: Use the Four Guardians’ Mirror in Meigu Lake to enter the Phantom Illusionist Ruins. Go through the shipwreck encased in ice, then ascend the staircase above Petal III to find the chest in a pavilion glowing green.
Where the Sundial of Impermanence is
The Sundial of Impermanence belongs to Quick Hand Zhang’s Treasures, and it waits in a chest inside the Phantom Illusionist Ruins. That snowy area opens when you press E at the Four Guardians’ Mirror in Meigu Lake. The chest is not out on the lake itself.
Look for a green-lit pavilion at the top of the stairs directly above Petal III of the Memory Flower of the Forbidden Blight Scripture. You reach it by the same one-way route that cuts through the wrecked ship frozen in the ice.
Route from the red-leaf tree bell
Rest at the Phantom Illusionist Ruins bell that looks like a red twisted bell-tree on snow. It stands beside a cluster of blue ice crystals under a big red-leaf tree, about 11 m from Petal V of the Blizzard Blademaster’s Memory Flower. Everything past this bell is one way, and you can’t walk back up to it.
Continue roughly 12 m to the chain suspended at the walkway’s edge. Once Space appears, press W and Space to latch on. It carries you across, then releases you automatically onto a long downhill slide, followed immediately by another.
At the bottom, travel about 70 m along the lower snowy slope to its end, then descend roughly 5 m to a small plank ledge. When prompted, press Space to catch the second hanging chain. Its swing deposits you on the deck of the shipwreck encased in ice.
Move across the deck toward the bow, drop roughly 2 m, then follow the deck’s perimeter to the broken section. A Frost Zither Maiden, club-wielding thugs, and a thug carrying twin axes guard this part of the deck.
The deck has no barrier at its edge; a fall from there is fatal.
A rope descends from the deck’s broken end to a lit plaza below. Position yourself beneath its upper end and look up; when Space appears, press it to grab the rope and ride down. Without the prompt, jumping from the edge will kill you.
Climb the steps from the plaza onto the raised walkway. Follow it to the bed of blue flowers at the foot of the next stairs, where Petal III of the Memory Flower of the Forbidden Blight Scripture lies.
Go up the stairs for roughly 27 m without jumping to reach the green-lit pavilion. Inside, the chest is in the room left of the doorway, with no enemies protecting it. Press E to open it.
Sundial of Impermanence card and hand-in
Once it’s yours, the item card appears under “Quick Hand Zhang’s Treasures” and reads “No one knows whose work it was, but the missing sections of the sundial were repaired with inlaid crystal. It does not measure time. It divines fate. And seeing where fate leads is not always a blessing.” It also says “A priceless treasure. It can be traded to Quick Hand Zhang for useful items.”
The sundial is one of three Meigu Lake treasures on the relic list “LOST TREASURES OF MEIGU LAKE”, alongside the Blue Blossom Compass and the Blight Astrolabe. Turn them in at the bell “Zhang’s Hideout (Meigu Village)”.
Another chest, containing Phantom Brew, is on a lower floor roughly 11 m beneath the Sundial of Impermanence chest. Reaching it does not involve the staircase leading to the pavilion.
