Absolution Gap (Volume 3) (The Inhibitor Trilogy, 3)

In the third book of the legendary Revelation Space trilogy, the last remnants of humanity realize that forging an alliance with a greater and even more mysterious alien force may be their only chance for survival.
The Inhibitors were designed to eliminate any life form reaching a certain level of intelligence -- and they've targeted Humanity. War veteran Clavain and a ragtag group of refugees have fled 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.
And 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 . . .
"Absolution Gap is a good as it gets, and should solidify Alastair Reynolds' reputation as one of the best hard SF writers in the field." -- SF Site