He looks like Kel'Thuzad to me.
My guesses are either one of these:
Ner'zhul
Arthas (Evil)
Lich King (Post Arthas / Ner'zhul merger, perhaps even a bit of Bolvar will be in there because he had the helmet for quite a while)
He looks more like he's wearing something similar to the Helm of Domination. Definitely nothing even remotely close to Kel'Thuzad.
I'm not convinced it's anybody we already know, though.
That art definitely makes it look like he could be KT. Interesting, I wouldn't have guessed that until seeing that art.
my bet
If he's like the other Covenants he's probably as old as the Shadowlands.
Mueh'zala I bet is one name he's known by
FFXIV - Maduin (Dynamis DC)
Arthas....
I hope he's actually none of the previously established characters and is just sort of his own character who exists independent of all of them
The Jailor is ancient.
He's none of the characters we have seen previously.
Formerly known as Arafal
Could Sylvanas be the Jailor even? I am not convinced about this, but hey, she jailed Bolvar in chains with her new unknown magic powers.
Took the photo into Photoshop to try and brighten it up a bit, all I could get is a little more of the crown on his head and he seems to have the same gem as Kel'Thuzad and the Helm of Domination.
And that is something Ion mentioned a lot, unknown entities from before the Titans. This expansion looks exciting because it seems to bring a change to the game play and a new Lore which was never really that mentioned, but it has always been there.
Exciting times lie ahead.
Looks like void walker eyes so Warlock gets a...get out of jail free card???