I've finally encountered the ebony warrior...and wish I hadn't.
To be honest, I'd seen the name bandied about but thought nothing of it. I even thought people were referring to the characters I sometimes met on the road: you know, the ones in full ebony armour and wielding magika as well as being handy with bows and swords.
How wrong could I have been!
I've just had my backside kicked halfway around Skyrim by him, four times. I lasted about a minute the first time. My last effort was about six minutes of hell before he blasted me into oblivion. He makes Legendary dragons look as fierce as hamsters.
I've now looked at videos for how people defeated the guy. Most of them did it by magika and dual-casting but my magika is just about good enough to light my way around dark caverns. The one bloke who beat him with a sword used grand healing like it was going out of fashion and he was only playing on expert difficulty. I've no chance of that. My magika isn't high enough, nor would it recover quickly enough. Another bloke blew him down the cliff and hit him with fireballs from safety while Serana drew his fire and took the onslaught below. I'm not using a follower.
I went at him with my best Daedric armour, Spellbreaker, an enchanted dragonbone sword and a batch of restore health potions. I had him down to about twenty percent health or so a few times but the b***** just kept getting up again. I have dragonborn force and hit him twice with the full three words of Unrelenting force twice in two of my all too brief encounters. Did he disintegrate? Did he heck. I was hitting the bloke for thirty seconds or so while he was lying flat out on the floor and he still wouldn't die. I tried marked for death on another occasion with the same sorry outcome.
Edit:
Victory is mine (on master level, anyway).
I made a mace with fire and shock damage on it instead of using the sword with absorb health and stamina. I thought that might work better since he was using the frost cloak. I Hit him a few times first with the Dawnguard exploding bolts of fire then waded in while the dragon was attacking him. I got a few bashes in before the dragon took off and he turned on me, then just used the shield/mace combo relentlessly (no power swings with the mace, just swinging repeatedly as fast as possible). I needed two potions of ultimate health and down he went...I still wouldn't want to have a go on legendary difficulty.