Chasing Shadows

Enter the Fatui hideout: Teleport to the Statue of the Seven near Dawn Winery and head southwest to the cave entrance.

Interrogate the guard: Enter the Domain and control Diluc. Open a Common Chest to the right. Proceed across a rotating bridge (use Kaeya's Skill to freeze flamethrowers if needed) and find another Common Chest past a broken fence. Defeat Electro Slimes and head upstairs to find a Fatui Guard. Tell him you're after the Holy Lyre, and he'll reveal it's locked up further inside.

Interact with the device to open the door and proceed upstairs.

In the large room, check the smaller rooms.

First room on the right: Answer "_We are goodwill ambassadors from the Snezhno-Mondstadtian Association_" and "_We’ll recommend you to the Association_" to avoid fighting an Electro Cicin Mage.

Far left room: Investigate points to spawn Pyro Slimes (use Kaeya's Skill), but the key is not here.

Middle right room: Fight Electro Cicins to open a Common Chest.

Room upstairs: Answer _“Give up. No-one’s going to rescue you”_ and anything else to avoid fighting a Wooden-Shield Mitachurl. Use Diluc's Pyro attacks to break its shield if you must fight. You will obtain the Hidden Key.

Open the sealed door: Return to the main room and interact with the device in the center to open the far door. Proceed and interact with the device to activate the elevator to the next floor.

Defeat the Agent and retrieve the Holy Lyre der Himmel: Approach the Holy Lyre, a cage will drop, initiating a boss fight against Fatui Zamenhoft.