This is called "Chronicle: River of Souls, Chains of Death" in the Chronicle window.
You will appear to be an Endless Court cultist. This will make some mobs inside non-aggressive to you (at least at first).
The Soul Cipher
Plutonus has discovered a Soul Cipher, an ancient Shadowstone machine that allows the user to identify specific souls within the Soulstream. Without one, it is impossible to pull out only the souls you want to find. The machine requires Dark Energy to power it so you are sent to gather three pieces for Plutonus. With the energy he finds two special souls Alsbeth will want. The first is Gaurath, a butcher that murdered whole villages before he was brought down. The second is none other than Zareph Mathos, the last True King of Mathosia.
The Soul Lens
At The Soul Lens, Alsbeth draws Gaurath in the form for a skeletal drake but he refuses to serve, saying he has served and died and will not do so again. Alsbeth commands you and your companion to beat him into submission. When he flies up, run around while purple circles chase you around the platform.
That done, Alsbeth summons her real prize but she does not realize that you are foe, not friend. When you stand next to Zareph and your disguises are ripped away, Zareph rises, summons an exit portal for you, and attacks Alsbeth! Your part here is done, there is nothing you can do to aid him further...