Could this be "Twilight of the Gods" by John C. Wright? This was published in 2009 in a collection called "[Federations][1]" edited by John Joseph Adams. I have not read it myself, but according to an [interview][2] with the author:

> The story springs from two roots. First, this is my attempt to tell
> the story of Wagner’s Ring of the Nibelungs in space, complete with
> elfs and dwarfs and mortals and immortals. Second, this is a sequel to
> my first two short stories sold, NOT BORN A MAN and FARTHEST MAN FROM
> EARTH

which fits with the Wagnerian theme you describe. A synopsis is given as:

> TWILIGHT OF THE GODS is a tale of a multigeneration warship that
> continues in the interstellar fleet action in which she is involved,
> long after the mortal officers, crewmen and marines have forgotten the
> meaning of the titanic eon-long battle around them. Due to an invasion
> of the lower decks by enemies, and the mutiny among the officers,
> control of the long-quiescent battlecomputers is lodged in a single
> set of codes carried on a ring—as far as the crew is concerned, this
> is a magic ring, and whoever possesses it becomes the captain of the
> steel universe around them. Conflict erupts when a lone wanderer from
> the heavy outer decks of the world-ship appears, bearing the fatal
> ring, and seemingly loyal to the ancient, long-lost sailing orders of
> the forgotten generations who launched the mighty warship. For he
> believes, although he has never seen them, that there are other
> warships beyond the steel walls of the world, friend and foe alike.

which fits with the generation ship having an internal civil war.


  [1]: https://www.goodreads.com/en/book/show/6277570-federations
  [2]: http://www.scifiwright.com/2008/12/interview-twilight-of-the-gods-and-other-matters/