I agree with kwak76 about the Legion. If you really think about it, they are ghosts, spectres who are mysterious figures, they aren't meant to be understood. They're another phenomenon of the Warhammer 40k universe that doesn't make sense, but still works, and that's why they aren't given a big presence. Also what yo're missing is that they are there, almost too much; The Damned Legionnaire that stalks Kersh and other characters throughout the novel is almost constantly there, watching over things and generally being mysterious, which I think is what Rob intended for the Legion. I read it on kindle so I don't know about the blurb, but tbh I was very impressed with the the novel and I actually prefer Kersh to the Legion. Kersh is like Sharpe in 40k, he's a bastard, condemned by his Chapter and fighting to prove his worth. Kersh my second favourite BL character now (after Thanquol), and I hope he gets more material written about him.
My sentiment exactly. Sanders handled this novel perfectly in my opinion and did indeed give us a truly great character in Kersh.