The Tsaesci which are another creature of the hist, though its said they have no birth sign so what are they?
Tsaesci have nothing to do with the Hist.
Tsaesci is "Snake Palace", once the strongest power in Akavir (before the Tiger-Dragon came). The serpent-folk ate all the Men of Akavir a long time ago, but still kind of look like them. They are tall, beautiful (if frightening), covered in golden scales, and immortal. They enslave the goblins of the surrounding isles, who provide labor and fresh blood.
This is pretty much all that is known about them, even though they have ruled the Empire for four centuries in the Second Era (the Potentate). But they're definitely not related to Argonians. The Akaviri are a special story. If you want to look a bit more into the Tsaesci, their http://www.imperial-library.info/content/tsaesci-creation-myth-and-we-ate-it-become-it is very interesting.