I think we are missing the part about thane talking about the Great Pact and that he addressed Ausar as a traitor. Seems there is a pact signed by all deathless together to imprison the worker of secrets?
I think Ausar might be evil in the past and the worker of secrets is out to defeat him. Then he captured the WoS and signed a pact with all the deathless to imprison him. The WoS gave the IB to Raidriar to finish his mission to defeating Ausar, which he did on the plain and Ausar was thought to be disappeared while planning his revenge by his "Bloodline"?