Tanaas asked you to participate in the Divine Fortress battle and talk to Zaken, who would tell you more on how to destroy the Divine Fortress Exterior Gate. Zaken told you that you need to obtain the artifacts near the fortress to power the Asmodian siege cannon. To accomplish that, you needed to kill the Divine Fortress Artifact Protectors first. When you defeated the Artifact Protectors and returned to him, he said that the Divine Fortress would soon fall into Asmodian hands.