Michael Cobley: The Orphaned Worlds [2011] paperback

Darien is no longer a lost outpost of humanity, but the prize in an intergalactic power struggle. Hegemony forces control the planet, while Earth merely observes, rendered impotent by galactic politics. Yet Earth’s ambassador to Darien will become a player in a greater conflict as there is more at stake than a turf war on a newly discovered world. An ancient temple hides access to a hyperspace prison, housing the greatest threat sentient life has ever known. Millennia ago, malignant intelligences were caged there following an apocalyptic struggle, and their servants work on their release. Now a new war is coming.