There is a cave called Sanni under Dagon Fel on an island and there are some Telvanis there and a prizoner Khajkit called Jo' Thri-Dar.

Is he of any use. I talked to all the people there and the salve but nothing seems to happen.

Is there any quest there at all anyone knows about?

J' Othri-Dar seems to be a prisoner in Sanni, because his attack is only 30. He lacks any non-standard dialogue, which leads me to believe that the devs might have at some time intended to give him unique dialogue, but forgot.

Go ahead and kill hom or whatever suits suits you.