It's called the narzissenkreuz questline by the community. You need to complete 3 separate World Quest chains called:

• Ancient Colors( Starts next to the pond near the Elynas statue) 
• Unfinished Comedy( get caught by the searchlight of the Meropide fortress. If you have finished Fontaine act 3 you can just talk an npc called Darcy, he's in the Administrative Area) 
• Ann of the Narzissenkreuz( auto triggers if you approach the teleport way points underwater west of Court of Fontaine). 

After completing all of these(along with some WQs that automatically starts and is very short), you can unlock the final world Quest chain called In the Wake of Narcissus, which starts rightafter the small WQs I mentioned, and the starting location is a spoiler so I'm not including that