Hylas has moved his most capable officers and nobles out into the Plane of Life. You managed to stop the majority of his forces from leaving Telara, but Duke Ellisar left our world and has been in this mystic realm for quite some time. We need to know what has happened to the Aelfwar that left our world, and most importantly what they are trying to accomplish.
Take Diona's Book of Aqueous Rituals and anchor Duke Ellisar's spirit before you slay him.
When one gives up their mortality in exchange for power of the elemental realms, one becomes vulnerable to the tools we have against the planar beings. I will now condemn Duke Ellisar to the Vault of Heresy. He will have time to reflect on how his abandonment of the gods and his soul in pursuit of power led to his eternal imprisonment.