Serna has been my favorite XML editor for years for real documentation work. Coincidentally I checked today the stage of the opensource development and found it had disappeared from their websites. As far I understood the Serna company been sold and the new owner had closed not only the opensource activity but the free (as beers) version availability.
Then I've searched for any copy of the open sources and seems to be available at Bitbucket: https://bitbucket.org/syntext/serna-free.
So I'm writing this for helping others to find it.