|The title of this article is conjectural.|
There is little information concerning these particular entities - general information on their kind may be found under Spirit, while their actions while in control of the human Carsaib, are described in the article on the Shade, Durza, and the general properties of possessed sorcerers in the article on Shades.
The spirits residing in Durza/Carsaib were three rather strong and particularly malevolent representants of their race. As most spirits, in their natural form they were ever-changing orbs of unknown energies. However, when Durza was destroyed and the spirits left his body, they looked more like clouds of shifting darkness. The reason for this difference is not known, but presumably it is due to the unnatural confinement within a physical body and its sudden end. Arya once said to Eragon that spirits can assume various forms according to their desire.
The origin of spirits in general is not known, however, Eragon Shadeslayer managed to learn from Durza's mind the story of how these particular spirits came to be in Carsaib's body. Carsaib was the son of nomads living somewhere along the edge of the Hadarac Desert at an unspecified time before the events of Eragon, having been banished from their tribe because of some unknown transgression of his father's. His parents were killed in a bandit raid and he was adopted by and became the apprentice of a travelling sorcerer by the name of Haeg. For some years, Haeg and Carsaib travelled together and the old man taught him, albeit reluctantly, the secrets of sorcery. Then one night, however, Haeg and Carsaib were ambushed by more bandits and Haeg was killed. Carsaib, being orphaned for the second time, summoned the most powerful spirits he could in the hope of exacting revenge on his master's killers. However, being still only a boy and a novice sorcerer, he was unable to control them and was possessed and lost to their will. Thus, Carsaib became the maroon-eyed, red-haired, white-skinned, evil-incarnate Shade, Durza.
The spirits gave the newly christened Durza not only his strange appearance and new personality, but also superhuman strength and speed, near-immortality and command of incredibly powerful magic, beyond even the abilities of most elves. It is unknown how and when the 'Durza' personality evolved, but presumably it takes some time before a freshly-created Shade comes to consider himself a single new entity, as the Shade Varaug, slain by Arya and Eragon at the end of Brisingr, comported itself more like a group of entities trapped in an alien body and did not appear to have any agenda beyond wanton destruction. Durza, on the other hand, could pass for human as far as his mentality was concerned, thinking of himself as a single being and entertaining intricate plans and plots to ends other than freeing himself from his flesh.