User blog comment:Jimeee/Dragonborn officially released on Xbox Live/@comment-5179958-20121213195226/@comment-5060344-20121217012643

If you read the Ahzidal's Descent (one way to start the quest off as well) it says that he was born in Saarthal before the Falmer sacked the city. He did not arrive with Ysgramor and the Companions, but he did seek them out when they arrived and enchanted their weapons (amongst other things) as they drove the Elves out of Skyrim. If you want more lore on him then the aforementioned book is the best place to look, he's probably the most detailed Dragon Priest after Miraak. In short though, he was obsessed with power and tried to absorb all of the magical knowledge he could to this end, which eventually drove him to madness.