Race: According to
The Real Barenziah he is part Bosmer and part Dunmer, with possibly some other mixture of races. Judging from his appearance, he seems to be primarily Bosmer, though the Dunmer part of him comes through quite a bit as well.
History: Not much; about all there is for before Arena is that he gained the Staff of Chaos by seducing Barenziah and tricking her into retreiving it for him. Reading the later chapters of
The Real Barenziah gives just about all that's known about him. I don't believe his birthplace or how he got the position of Imperial Battlemage are known at all.
Motive: Why did he trap the Emperor and take his place? That's the big mystery. Battlespire says he was working with Mehrunes Dagon, so he may have been trying to help him, though in that case he could have just killed the Emperor, kept the Amulet himself, and thus caused the Dragonfires to go out - though quite possibly that wouldn't have worked at that time. More likely he just wanted power, though that sounds too simple; I'd prefer him to have something more fun for a motivation.