This quest is fairly easy, it consists of a lot of story telling. First you have to kill a number of Penitus Oculatus Agents. They are fairly easy to kill as they are weaker than guards. I even used a bow and arrow in close combat and was easily able to dispatch them.
Inside you find Nazir fighting Arcturus. I found that satying out of the fight and using a bow and arrow to get shots on Arcturus was the easiest way to kill him. Don’t worry about hitting Nazir, he can’t be killed and he won’t turn on you if you shoot him too much.
To escape the fire you get into the coffin of the Night Mother. I am not sure how Nazir and the girl survive, but they do and they pull you out of the water. Afterwards you talk to Astrid and she eventually asks you to kill her with the Blade of Woe.