Descent of Angels

The first five books were each better than the previous one, building on and fleshing out a cohesive story. This one hits like a huge speed bump. It's kind of like two separate stories smashed together (or maybe one and a third) neither of which have any relation to the overarching story of the series. They're kind of lopsided as well; the first “half” drags on and you start to forget it's a Warhammer 40K story at all, while the second half feels more like the rest of the series with some interesting warp horrors but is really rushed.