The Black Rift — Michael G. Thomas

The Black Rift is the ninth and final book in the epic 'Nexus' series, part of the growing Star Crusades universe that chronicles humanity's first steps away from Alpha Centauri and beyond.

The Prophecy of Fire has revealed itself in the shape of the great machine enemy, the Biomechs. Their limitless numbers of ships, soldiers, and creatures are ready for the war to end wars. Each of them waits patiently for their chance to come through the unstable Black Rift under the command of their remarkable new ally. A glimpse of this world-ending force has already been seen, and it has led to a series of disasters on many worlds. Even the stalwart defenders of humanity begin to look inwards, and for a leader that can save them from the fury of the machines. Few doubt the Biomechs' resolve, but there is now little remaining to guard the ancient gateway to the domain of the enemy.

As the allies wait for the arrival of the hated machines, they are forced to defend the burned out worlds of Eos, Spascia, and Helios Prime. The outcast Biomechs of the Nexus hear the call of their kin, and hurl themselves at every ship and soldier in a final bloody assault that could end the war in a matter of days. The allied forces of six different races cling to their trenches and earthworks in massive sieges, while in space the fleet fights bloody engagements that leaves hundreds of ships in ruins. 

The inhabitants of the Nexus finally realize that the Prophecy of Fire was true, and when the Black Rift finally opens, there might be little left for the few remaining warriors to fight for.