So ghoul->priest->A.Sleeper. It's all part of the evolution they go through?
Yeah. Don't take my word on this totally, but I always thought that basically, you can come to dagoth ur in two ways. Either his dreamsends turn you into a sleeper/dreamer, or you get corpus and go to the red mountain. In fact, I think that corpus is literally a
divine disease - it comes from the heart of lorkhan, and is what all of dagoths servants are made from. In most, this divinity is uncontrolled and warps the body and destroys the mind - corpus monsters. To those who serve Dagoth Ur, this divine spark is controlled and manipulated, eventually turning you into an ascended sleeper.
Ash slaves, the lowest level, seem to have no eyes, implying that perhaps they must remove them - this would increase their reliance on their new divine senses. Ash ghouls have lost their face (except their mouth completely), making way for the growths that develop in the ash priests and mature fully in the ascended sleepers.
Also, what ARE ash vampires? Chimer-turned-Dunmer? Why are they so tall, frightening(yes i nearly crapped myself after seeing the first on at the end of the hallway when you have to do the Urshilaku quest to acquire 3 items from the 6th house base. Gives me creeps thinking of it now), and what's with their teeth?
It's explained in game I believe, though its been a while since I did the main quest. I think they were House Dagoths nobles, Dagoth Urs lieutenants, etc. that he bound to the heart as he is.
Edit: Also. Why does Dagoth Ur/Ash Vampire have three eyes? Due to the mutations or to make it known to all that Ur and the Vampires are the highest authority?
In the real world, the "third eye" is a spiritual symbol. Dagoth Ur and the Ash Vampires are awakened, enlightened, whatever, and that shows it.