So I decided to dabble a little more into magic (I've only had two mage's, a Dunmer and Breton, and they didn't get too far), so I made a new one.

This guy was good, but was tainted by the powers of Oblivion, and became a Dark Mage. He wears an Ebony Chestplate and Ebony Boots, Nightingale Gloves and a Nightingale Hood. He wields a powerfully-enchanted Ebony longsword in one hand and casts flame spells from the other. However, I'm not sure where to take this guy character-wise and quest-wise. What would he do? He's no assassin, so the DB is out. He also has no interest in the Civil War, but he is determined to destroy Alduin, as he "Likes this world. I don't want it to end."

I have the Dawnguard DLC and will have Dragonborn by the end of the week (YAY!). What should I do with this guy?

Also, I think he'll have Serana as a follower, no matter which side in Dawnguard he chooses, although it's probably going to be the Volikhar.

P.S. For combat he uses exclusively the Ebony sword and his Ebony armour, and only when he needs to. Besides that, he will be purely magic. What skills other than Destruction would best fit an evil battlemage?

Maybe Illusion. I'm not sure how evil your character will be, but perhaps he'll like seeing enemies turn on each other.

Think Anakin from Episode III. Tainted by the evil but there's probably good in there, it's just been overpowered and he embraces the evil to bring him more power. He didn't intentionally become evil. His goal was to collect the Daedric artifacts and lock them away from the world. I'd say he's 80% evil, with that little voice of reason speaking to him rarely.

If you just made this guy, how do you already have ebony!? O_o

and yeah, I'd agree with illusion, it seems like a darker type of magic

if he's "dark" or even evil, don't rule out conjuration (with a focus on necromancy)

Well, just made as in this new arc in his story. The actual character's about two weeks old, but the evil has tainted him now.