What is known about him? I've read the section on UESP:Lore but it says very little. Is anything else known?

Yes. Have a look at the following texts:
http://www.imperial-library.info/dfbooks/b067_ebonarm.shtml
http://www.imperial-library.info/dfbooks/b022_memorystone.shtml
http://www.imperial-library.info/dfbooks/b007_oelander.shtml
(http://www.imperial-library.info/dfbooks/b100_complete_edward.shtml)

So Ebonarm used to be a person? A mortal who was later changed into a god like Tiber Septim? If not, which mythology did he come from? Nordic? The sources say very little about him, only mention him briefly.

Bretonordic.

An irrelevant, minor god.

Can't find him in the Pantheon page of uesp.net.


If he opposes the Daedra, how powerful is he?

A minor irrelevant god. Any capable hero can oppose the daedra. Any capable hero can defeat the daedra. Any capable hero can humiliate the daedra. There's a long history of daedric defeat by mortals. So, don't get so excited over Ebonarm.

Oh, but you just made me even more excited about him.

What if he finally finds a way to tackle with the Princes?

There are daedric artifacts that have already allowed mortals to tackle with the princes.

Which artifacts would that be?

Armour (Cuirass, now) of the Saviour's Hide is a main one. Along with the Spear of Bitter Mercy.

Can't remember any others right now.

Not very impressive, this Hide. It didn't to the job, Dagon still got the poor bugger.

I'm still interested in giving Ebonarm his own story.