The Heart of Ouroboros is one of three subquests that you’ll unlock during The Subterranean Trials of Drake and Serpent World Quest. After meeting the afterimage of Aru, you’ll be asked to find three Reins of Revival, one of which can be obtained from this subquest.
Start by following the quest marker to the altar in the Serpent’s Heart. You’ll find a mechanism that can switch between Whitenight and Evernight. Interact with it to set Enkanomiya to Evernight. The altar will be coated in transparent walls with symbols, which you can’t get past from the outside.
Enter the altar from the open area and interact with the Investigate spot to prompt a cutscene to play. The symbols on the ground will light up.
Related: How to complete the Enkanomiya prerequisite quest: Still Water’s Flow in Genshin Impact
Each symbol is paired with a number of dots, which indicate the order that the transparent walls should be activated. All you need to do is to walk past the transparent walls (from the inside of the altar, going outward) in the same order as the symbols on the ground.
So walk past the first symbol, head back inside, walk past the second symbol, head back inside, and so on until you finish all five. If you solve the puzzle correctly, a cutscene will play and a Chest will appear at the center of the altar. It contains the Quest Item Reins of Revival Fragment (Serpent’s Heart).
Check out our guide on another subquest required to obtain all three Reins of Revival: How to complete The Trail of Drake and Serpent in Enkanomiya Genshin Impact.