Facet of Grace

Load into The Lost City, then move backward past Micah-10 to begin Facet of Grace's puzzle.

After entering the large courtyard, make a sharp right to kill a group of enemies that includes a Gordian Weaver (you'll also see the prismatic chest here, but we'll come back to it). Doing so should say "First Omen is defeated" and "Omens appear" in the game chat. From here, run directly across the courtyard to kill a second set of enemies (marked two on the map above). After these, run back across the courtyard and make a shallow left to the position marked three above, and kill the enemies there.

Finally, return to the central courtyard area to kill two omens and a Tormentor, the last of which will drop a key for the prismatic chest (which is located back where the first group of enemies were).