The Damnation of Pythos by David Annandale

In the aftermath of the Dropsite Massacre at Isstvan V, a battered and bloodied force of Iron Hands, Raven Guard and Salamanders regroups on a seemingly insignificant death world. Fending off attacks from all manner of monstrous creatures, the fractious allies find hope in the form of human refugees fleeing from the growing war, and cast adrift upon the tides of the warp. But even as the Space Marines carve out a sanctuary for them in the jungles of Pythos, a darkness gathers that threatens to consume them all.