The Sadras-Dagoth War was a series of skirmishes in the province of Morrowind that occurred circa 4E 427 on the sub-continent of Vvardenfell, as well as the isle of Solstheim and the city of Mournhold. It was primarily waged between the resurfacing Sixth House of Dagoth Ur and the newly formed House Sadras under the leadership of the enigmatic Nerastarel Sadras. It lasted during and shortly after the Nerevarine's own dealings with House Dagoth.
Nerastarel was a Dark Elf who was originally a resident of Skingrad in Cyrodiil, before being the victim of a misunderstanding with the town guards and a cult of Necromancers, which resulted in him being imprisoned and his house being infested with undead creatures. However, the true circumstances of this event are unknown. Later, Nerastarel was pardoned for unknown reasons, and was sent to Vvardenfell alongside two other prisoners: Jiub from the city of Kvatch, and an equally enigmatic individual who would later be identified as the Nerevarine.
During a mysterious storm, Nerastarel escaped from the prison ship carrying the trio by diving overboard in the confusion and chaos. Allegedly, his ambitions were too great to be held back by whatever the Empire had in store for him. He managed to enter Seyda Neen without arousing any suspicion from the Imperial Legion soldiers posted there, and from there, went on a conquest throughout the island of Vvardenfell.
In the ensuing weeks, even months, Nerastarel had attained mastery of spellcraft unlike anything before seen in Morrowind, and his power and influence could rival even the most skilled Telvanni mages. Using an army of summoned Daedra (primarily Dremora and Golden Saints), he managed to rid several caves and ancient Dunmer strongholds of the outlaws and bandits that roamed them, claiming mastery over each of them. He later converted many of the citizens of Vvardenfell to join his cause, persuading them to leave to the leave the cities controlled by the Great Houses to populate his controlled territories.
As time passed though, Nerastarel's mysterious intentions developed a noble streak when it became apparent that House Dagoth's influence was spreading too far from Red Mountain, and the Blight epidemic was getting worse and worse. Thus, Nerastarel decided to form a new Dunmer House that would protect Vvardenfell from all domestic threats. His forces initially established themselves in the sparsely populated region of Sheogorad, where Sadras could grow uncontested and at a safe distance from the ash storms spewing forth from Red Mountain.
Ashlanders, hermits, outlaws, outlanders, and even Vampires pledged themselves to Sadras to live as their citizens. And the more people submitted themselves to House Sadras, the more people it meant defending.
The war begins
House Sadras expanded its territory, relieving control of settlements such as Gnisis and Dagon Fel from the Empire, and Nerastarel himself personally led assault after assault on the diabolical Sixth House, raiding their outposts and territories beyond the Ghostfence. The military strength of House Sadras consisted mainly of Orc mercenaries, as well as Nerastarel's own army of summoned Golden Saints and Dremora and several modified Dwemer Centurions. By 4E 428, Sadras later was reinforced by Nords from Solstheim, who brought with them tamed wolves and bears, which were previously never seen anywhere on Vvardenfell.
The battles were numerous and bloody, and several stretches of the island nation were occupied by Dremora and Golden Saints waging war against not only the Sixth House's Ash creatures, but the local wildlife. At the suggestion of Nerastarel's former cellmate Jiub, Sadras' military forces were instructed to treat Vvardenfell's entire population of cliff racers, which had an immensely bloated population and was spreading the Blight over the Ghostfence just by flying over them, as an extension of House Dagoth. Nerastarel, who himself developed an intense hatred of the cliff racers during his time on Vvardenfell, was only too happy to agree to Jiub's plan.
The later stages
As the Nerevarine led the assault on Dagoth Ur, House Sadras' forces followed in shortly after, killing many ash creatures and Corprus beasts that infested the land within the Ghostfence, around Red Mountain. House Sadras was exhausted during the struggle, but saw victory alongside the Nerevarine and the Great Houses nevertheless.
Even months after Dagoth Ur's defeat, House Sadras was still purging Vvardenfell of all remnants of House Dagoth, and treating or killing all creatures infested with the Blight and Corprus diseases. By 3E 429, Vvardenfell's countryside and waters became littered with hundreds of slain cliff racers until eventually all the cliff racers on the island were either exterminated or re-located to mainland Morrowind. It was Jiub that was credited in leading the campaign against the cliff racers, and was named a saint shortly afterwards.
Nerastarel recalled most of his forces apart from the summoned Daedra back to the Sadras-occupied Dunmer strongholds, and to Solstheim. The Nords and Orcs that served the house became more and more numerous, to the point where they wound up forming the majority of the lesser House's non-Daedric military strength, leading many to believe that House Sadras was simply an army of Nords from Skyrim instead of a lesser Dunmer House.
Following the Red Year of 4E 5, House Sadras was finally recognized for its deeds and was elected to replace House Hlaalu on the Grand Council of Morrowind, after the latter was stripped of its rank.