The mission takes place in the Cathedral of Hidden Depths, a sunken temple of Abaddon now occupied by Risen. Much of the area takes place underwater and in close quarters, so prepare appropriately. Enemies hit hard if they catch you unaware, but fortunately, Sayeh is remarkably tough and can withstand multiple enemies on her own. There are also a variety of useful environmental weapons scattered around the mission, sometimes inside of Orrian Chests. Of particular note, the quest marker is remarkably unhelpful in this mission due to the small area/levels/walls, and it's difficult to describe in text exactly where to go. You start in the submerged portion of the temple, eventually working your way up until you are near the surface. Search just a few feet below the surface until you find a small tunnel that is smaller than the others. This will lead you to the surface.
Upon arrival, you find that the Risen have beaten you to the temple. You will see a large hole in the ceiling leading to the next area, but the doors are closed. After defeating the group of Risen and the Risen Crab that spawn, head through the door directly in front of you. You will come upon a glowing Temple Orb and an Abaddon Shrine. After clearing the room, interact with the altar. This spawns a Devoted of the Deeps for you to defeat; it doesn't hit very hard except for Chaos Storm. After defeating it, it will turn to an ally, bow, and leave behind an Illusory Essence at the altar. Pick it up and place it on the temple orb.
After placing the Illusory Essence, a cutscene plays. The cinematic shows you the path to a now unlocked door. Head back to the starting area and swim up through the large hole in the ceiling. A group of three Risen will attack you from the now open door visible after you swim up through the hole. Head through and you will arrive at a chamber with orb and altar puzzle. Take a right past the temple orb down a corridor with Risen Mines, take them out with ranged attacks. Clear the room and activate the shrine, you will now face a Devoted of Illusions. Defeat him and place the Illusory Essence on the temple orb. You will be ambushed by a few Risen along the way back to the orb, some of which are the toughest in this dungeon and quite buggy and require use of the tools in the room. The Illusory Essence has some decently powerful skills to help ward them off.
This time, there is no cutscene to show you where to go next. You might find yourself apparently stuck in a room with a circular ramp above water and unable to jump up, the cause of much frustration with this dungeon. This is because you went too far. Go back and look in the central area, swimming along the upper walls. Compared to the other passages you have seen, the next passage is a much smaller open opening to the Southwest half obscured by some giant glowing purple barnacles. You'll know it's the correct one when some enemies come out on ground level; defeat them. Head through, and you will end up on the ground level of the temple; all combat from here on will be out of the water. You will see the shrine sitting on some scaffolding. Defeat the Devoted of Nightmares, it will use Chaos Storm often and inflict Fear, so bringing condition removal is a good idea. Bring the Illusory Essence to the temple orb is farther down the corridor. You will be again ambushed by a group of Risen at the temple orb. This will open a nearby door, follow the metal walkway into the next room.
You have reached the ground portion of the temple. The pool of water leads to the submerged portion you have been exploring up to this point. Take care not to fall in the water or you will have to go back around to get up. Defeat the two Risen at the top of the ramp and the Rotmouth Sucker that spawns. Head up the ramp into the next room. In this room, there are two shrines and orbs for you to complete. The Devoted of Secrets is a ranger and uses a shortbow, it will shadowstep, evade, blind, and poison you. The Devoted of Lies is a mesmer similar to the previous Devoted. Defeat them and place the Illusory Essence on the temple orbs. This will open the door to the final area. Head up the ramps to the next room, where the Altar of Secrets awaits.
When you reach the altar, three powerful Risen are already there: a Mouth of Zhaitan, an Eye of Zhaitan, and Risen General Azuldin. A cutscene plays and you must defeat Azuldin. He wields a greatsword, moves slowly and doesn't hit particularly hard. He uses a charge attack that smashes the ground (with a greatsword) and an uppercut attack, both attacks are easy to dodge. He will occasionally spawn five Gravespawn Grubs. He will causes the room to shake, sending bits of the ceiling crashing down. Once his health reaches 25% health, he will gain permanent might, swiftness, fury, and protection. After he is defeated, Zhaitan roars in fear, breaking the foundations of the temple and causing it to collapse. Escape through the hole; the room is conveniently located above the pool of water in the central area. Talk to Sayeh for a cutscene.
