Ahzidal's History
The man that would become Ahzidal grew up in the first Nordic city of Saarthal during the late Merethic Era. As a child, his aptitude for magic was evident to his tutors. As an adult, he surpassed all of them. With nothing more to learn in Saarthal, he left his wife and child to train under the Elven masters.
Vengeance: He returned to Saarthal several years later, only to find his hometown had been sacked by the native Snow Elves who had lived in peace with his kind for many years. In his rage and grief, he took for himself the name of Ahzidal, meaning "embittered destroyer," and swore to exact terrible vengeance against the Mer. Although he would come to fulfill his promise of revenge, it cost him his sanity.
Years passed, and Ahzidal would travel the lands and study ever deeper into the arcane arts, biding his time. From the Dwemer, he learned the seven natures of metal and how to harmonize them; from the Ayleids, the ancient runes and dawn-magic. He sought out knowledge from the Snow Elves, Chimer, and Altmer, hoarding what he could from each, all the while plotting how he might turn that knowledge against them.
When Ahzidal learned that Ysgramor had returned with a war band of the greatest warriors and hunters of Atmora to reclaim Saarthal, he immediately sought them out and offered all of his knowledge to the Five Hundred Companions. Imbued with the magical enchantments that Ahzidal bestowed upon them, Ysgramor's elites utterly decimated the Snow Elves occupying Saarthal, and would eventually end the dominion of Mer over Skyrim. The oath of the "embittered destroyer" was fulfilled.
Descent into Madness: Yet, in his lust for vengeance and power, Ahzidal became ever more corrupted and he eventually fell to madness through his obsessions. Not satisfied with the knowledge of either Men or Mer, he next sought out the secrets of the dragon-runes, which led him into the order of Dragon Priests. However, even that failed to suffice in his hunger and Ahzidal later turned his focus on the planes of Oblivion, at which point he subsequently came under the thrall of the Daedric Prince of Knowledge and Fate, Hermaeus Mora.
Ahzidal finally lost himself, falling prey to his own pursuit of perfection. He fled to Solstheim, only to be sealed away by the Dragon Cult within the depths of Kolbjorn Barrow together with the last of his relics, and the once great human became forgotten by history.
Return and Final Demise: A millennium later. Ahzidal saw his chance to escape in the form of Ralis Sedarys, a Dunmer treasure hunter wanting to excavate Kolbjorn Barrow. Ahzidal corrupted his mind to release him from his tomb.
Eventually, the excavation caught the attention of the Last Dragonborn, who had come to Solstheim to save the island from another Dragon Priest, Miraak. Ahzidal had Ralis convince the Dragonborn to fund the excavation, who agreed and continued to have investment after investment into this project. As each investment woke more of the Draugur that once served Ahzidal. The more the Dragonborn was forced to both kill and invest more money in order to convince more people to mine for the excavation (the miners were being killed each time they uncovered more of the Barrow).
Eventually, the miners uncovered enough of the tomb to reach Ahzidal, who had Ralis and the Draugr kill everyone. The Dragonborn, after fighting the last of Ahzidal's undead minions, came across Ralis using the blood of the miners to resurrect the Dragon Priest himself.
Knocking Ralis out, Ahzidal engaged in a bloody battle with the Dragonborn, who finally managed to kill the Dragon Priest, ending his madness once and for all.