Absolution Gap by Alastair Reynolds

They are ancient killing machines, designed to locate and destroy any life form reaching a certain level of intelligence. Now, stirred from eons of sleep, the Inhibitors have descended on their latest target: Humanity." "The first wave of Inhibitors has sent war veteran Clavain and a ragtag group of refugees into hiding. Their leadership is faltering, and their situation is growing more desperate. But their little colony has just received an unexpected visitor: an avenging angel with the power to lead mankind to safety - or draw down its darkest enemy." As she leads them to an apparently insignificant moon light-years away, it begins to dawn on Clavain and his companions that to beat one enemy, it may be necessary to forge an alliance with something much worse.