When entering Dune, Esan will be with Shazah and say, "Yes! This should work! The Fang is attuned with the Daedra's essence." Speaking to Shazah, she will say that after seeing the Daedra, Esan raided the temple vaults in order to retrieve the Sacred Khajiiti Fang Talisman.
After the portals at Dune's entrance are closed, she will send you to rendezvous with Esan, who has gone to check on the second portal near the Mages Guildhall. After reaching the entrance, Esan will be lying wounded on the floor. He will explain that Magister Bayya betrayed the Guild for Javad Tharn, and mortally wounded him before heading for the inn. He says Bayya has sealed the Hall using a totem, and tells you to kill her, shortly before dying of his wounds.
"Good to meet you. This one is honored to be working alongside the Champion. Our next Mane!"
After speaking to Shazah:
"The destructive act unleashes their corporeal form, yes? Untethers them from Nirn. The attunement Esan has put on the Fang draws forth power from that tether, harnesses it for our needs."
At the entrance of the Mages Guildhall:
"We... we have been betrayed."
- What happened?
"Esan came as the Champion bade. He came to speak to the Guild. But the Magister... turned on Esan. They have killed me."
"Bayya has a warding totem. Used it seal the hall. Always... always said it was protection magic. But the Mages. They're trapped in the Hall. Kill Bayya, and destroy the totem. Save the Mages. And this city."
- I will, Esan.