Board Thread:Warhammer 40k General Discussion/@comment-9558536-20130517084538/@comment-4264245-20130517155747

Found it! "The Gates of Terra" by Nick Kyme. Arcadese is now a Captain and is in charge of a host of Ultramarines who are manning an orbital defence line called "Ardent Reef":

http://apocalypse40k.blogspot.com.es/2013/03/new-black-library-horus-heresy-releases.html