Is there anything known about the Citadel of Ebonarm in the Fourth Era? There is a theory that the Ebony Warrior might have a connection to Ebonarm. I think the Citadel might have taken over the Fighter's Guild in Hammerfell following the Oblivion Crisis, or definitely after the Great War. Also, why is Ebonarm the enemy of all Daedra except Shegorath, and hates Stendarr and the Cabal?

A god of war probably wouldn't make nice with the god of mercy all things considered. As for the rest...hard to say. There is the possibility that Ebonarm has just probably fallen off to the wayside (both in and out of the universe), if not regulated into obscure spirit worship. No clue as to the citadel though, since it never really comes up in the book on the Great War. Also, the Cabal IIRC preformed Daedra summoning for the Mages Guild, so the reasons for that are pretty obvious.

Fun fact, the Ebony Warrior isn't the first bonus boss we've had http://www.uesp.net/wiki/Tribunal:Gaenor