A'dal has asked you to slay Harbinger Skyriss in the Arcatraz of Tempest Keep.
Harbinger Skyriss is the heart of the problem in the Arcatraz.
He corrupted Warden Mellichar and made him release many of the prisoners. It is only a matter of time before the Harbinger himself is let loose.
<name>, you cannot let a servant of the Old Gods live!