No, seriously. I'm looking for a RP background for my great house dagoth playthrough. I've never joined Ur before, so this should be fun!

This man should be anti-imperial, greedy and should have an enormous hunger for power, even to condemn a whole island to damnation for more power. Some kind of dark mage?

As far as I know, he should follow the directions of Caius Cosades up to a certain moment. Why should he care about doing this job for the emperor, the man he despises?

What kind of skills would fit this man? Race could be an outlander dunmer, breton or altmer.

I've never done this sort of playthrough, but if I were to, I'd play someone who has a disdain towards the Tribunal (for legitimate reasons, e.g. War of the First Council) and therefore refuses to acknowledge the Armistice they've made with the Imperials under false pretenses. Doesn't have to be a bloodthirsty maniac, although a authoritarian/nationalist/proud streak does help.

Skill-wise I'd possibly go for long blade, axe, spear, and/or destruction, as those seem to be sort of traditional Dunmer fighting styles. Light armor and unarmored outfits seem to fit House Dagoth the most. Speechcraft is also a fine choice, in my opinion.

Spoilers galore:

Possibly an npc such Orvas Dren. Most Dunmer who join the Sixth House aren't in their right mind. Dagoth Ur's goals would be aligned with someone who wants Morrowind or all of Tamriel in time, to be free of all existing kings and gods in a new divine state. This involves destroying the Empire and the Tribunal and that's what appeals to Dren, but I think it's a means to an end. As for mercenary motivations, it would either be some sort of gain from living in an independent, post-Tribunal Morrowind or gaining a place at Dagoth Ur's side. Otherwise, it's not really some you'd do for a selfish powergrab.

For poltically motivated character, skills concerning infitration and sabotage would be useful for fighting against Tamriel's biggest authorities. So that could be like the skillset of an agent or nightblade. For spiritually motivated character, a mage, sorceror, monk or healer might be expected.