Board Thread:Skyrim/@comment-107.7.27.190-20130329180724/@comment-24.197.208.195-20130622061715

I think that he lost his spirit. The humiliation and madness of being chained to the ground probably just took his spirit. This may sound odd, but I think that part of their immortality is their gift of flight, so being held down for years may have made him mortal. His head was likely just taken off after death as a trophy.