In the southwest islet of The Serpent's Heart, you'll encounter a puzzle that can only be accessed during Whitenight. Solving it will grant you a rare Luxurious Chest!
Start by heading to the area circled in the map below—make sure to jump down to the platform under the cliff. You can only enter during Whitenight, but if you forget to change the daylight setting in advance, don't worry as there's a day-night switch mechanism on the left.
Upon entering, you'll find a Suncore, a triangular mechanism surrounded by stone walls, and a Luxurious Chest locked behind a Whitenight barrier.
Attack the triangular mechanism twice to prompt the stone walls to move twice as well. If the stone walls don't move or only move once, just keep trying until they move a total of two times.
Once that's done, approach the Suncore to obtain its blessing and head outside, where you'll find a day-night switch mechanism. Interact with it to turn Enkanomiya into Evernight. Once it's Evernight, the entrance will be blocked. Simply enter through the other entrance on the right (be careful not to fall—the pathway is narrow).
The blessing of the Suncore will activate the triangular mechanism. Attack it twice until the stone walls move twice, and you'll be able to access the unlocked Luxurious Chest, completing this puzzle.
Check out our guide on How to solve the Evernight Temple barrier puzzle in Enkanomiya in Genshin Impact to switch between Whitenight and Evernight like a pro.