Board Thread:Warhammer 40k General Discussion/@comment-24821739-20140604204305/@comment-24944355-20140612002145

Magnus would use sorcery to overwhelm Russ, he would not be so foolish as to take on Russ in a fair fight again. Also I said I think Magnus imprisoned Russ so as to punish him for forcing him into his current situation, which does not mean he isn't secretly aiding The Emperor. By imprisoning Russ instead of killing him Magnus gets to punish him and keep him alive to unleash at the right time.

As a side note The Emperor has the power to unleash any - or all - of the surviving primarchs at any time, one of his living saints could easily heal Guilliman for instance. I believe he is waiting until he is ready to take on the ruinous powers in the warp before bring his surviving sons back to smack down Abbadon & co so as to improve humanity's chances following The Emperor's final rise to godhood in the warp.