Black Library has released two Betrayal at Calth novels.
There are two separate novels. One from each sides perspective.
The complete story of the battle between the Honoured and the Unburdened beneath the surface of Calth. Two old allies, now bitter foes, clash in the darkness, and only one will survive...
It's both sides of the story in one gorgeous box set. Two interlinking stories set in the caverns beneath Calth see the Ultramarines and Word Bearers battle to the death. And it might just be everybody's death, because there's something else in the darkness waiting to be unleashed from beyond the veil...
I ordered all of them!