His name is Sam Guevenne, he wears a black mages robe, and under normal circumstances (when there's no fight going on, which I tested after I "discovered" him) he asks you to take part in a drinking contest against him. I haven't tried the drinking contest itself, I just wanted to see his dialog. Now the thing is I went back to the previous save when the fight inside the Bannered Mare was still going (me, Lydia and a Battle-Born guy fighting with me for some reasons). So that guy, Sam, he was just running around trying to simply avoid the actual fights. I was curious, so I attacked him (shouted at him, I used the first shout we get from that scripted dragon at the western Watchtower). Bad move...
I won't spoil too much even with the spoiler tag, but if you guys ever attack him, be prepared to die, unless you're level... like... 30+? I'd imagine, or at least around that. When I saw what the guy was and what he could do I had a good laugh, he's ridiculously powerful (well, both directly and indirectly, you'll see, if you try). Good thing that saved game is just there for some tests (just checking around if specific fights can force the game to CTD since I'm doing some stability tests).