There are many powerful lieutenants among the Mawsworn, but there is one who is particularly important to the Jailer.
Far beneath the Tremaculum lives the Jailer's prime interrogator, Grathalax. Power is fed to him via conduits throughout the area.
His role is to make the process of anima extraction more efficient. His methods are... unkind.
I believe we would both benefit from the end of his existence.