Short story: Generation(?) or battleship based on Norse myth culture. Read in last 15 years, perhaps in The Year's Best Science Fiction edited by Gardner Dozois. Not a sure if it was a generation ship or just a large battle cruiser on an extended trip that had been damaged. Command structure adopted elements of Norse mythos or parts of Wagner’s Ring cycle. Ship was still at war with another ship or internal civil war, I think. And maybe the story title was unusually grandiose or operatic. Wading through story titles in YBSF, volume by volume, but no trigger yet.
Could this be "Twilight of the Gods" by John C. Wright? This was published in 2009 in a collection called "Federations" edited by John Joseph Adams. I have not read it myself, but according to an interview 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 theme of a generation ship having an internal civil war.
It has also been published in "The Year's Best Science Fiction: 27th Annual Collection", edited by Gardner Dozois, so it seems a good candidate for the story you remember.
Partial match to Northworld by David Drake, in which the eponymous planet has lost its all-powerful leader and is missing from the "Consensus". A human police officer is recruited by the non-human controllers of the Consensus to find out what happened.
The officer is sent to the world where he finds a blending of mental abilities that enable him to move outside the world into other realms, which are controlled by god-like figures and are competing for resources.
At some point the Norse sagas are involved. I think it was a re-telling in a sci-fi context.
You can read it for free here