Absolution gap - Alastair Reynolds

Mankind has endured centuries of horrific plague and a particularly brutal interstellar war . . . but there is still no time for peace and quiet.
Stirred from aeons of sleep, the Inhibitors - ancient alien killing machines - have begun the process of ridding the galaxy of its latest emergent intelligence: mankind. As a ragtag bag of refugees fleeing the first wave of the cull head towards an apparently insignificant moon light-years away, they discover an avenging angel, a girl born in ice. She has the power to lead mankind to safety, and the ability to draw down their darkest enemy.
And on a planet where vast travelling cathedrals crawl towards the treacherous fissure known as Absolution Gap, an unsettling truth becomes apparent: to beat one enemy, it may be necessary to forge an alliance with something much, much worse . . .