Guillaume was named the Battle King in the Novella, so it must be someone who is in Ultramar with him (in the Imperium Secundus).
My guess is Sanguinus, Fear to Tread tells us that he was "lost" for several years because of the Signus incident. Also, for all we know from Betrayer und Fear to Tread, he is trapped in Ultramar now. Lorgars Spell makes sure noone will get out of the 500 worlds, but somehow Sanguinus has to make it to Terra before Horus arrives there.
The cabal might be able to help him make it, and that might also explain why the UM won't join the Blood Angels: Guillaume would never ever trust some bunch of aliens to savely deliver them to the throneworld!
Last edited by DjdaForce; 02-13-13 at 09:14 AM.