Continuum (Riven Worlds Book One) - Amaranthe, #14

Humanity has seized its destiny among the stars. But space remains vast and untamed, and nothing has prepared us to face the dangers rising from the deep shadows of the void. Fourteen years after The Displacement flung humanity into a universe teeming with alien life, a tenuous alliance has taken root among humans, Anadens, and numerous other species. The wounds of war and revolution have begun to heal, peace and prosperity are within reach, and the architects of The Displacement, Alex Solovy and Caleb Marano, are enjoying an idyllic existence on the living planet of Akeso. But growing troubles fester beneath the surface of this alliance.
An upstart species offers allegiance with one hand but readies weapons of mass destruction with the other, while the Anadens, leaderless and adrift for years, increasingly refuse to play by humanity's rules. As tensions simmer, Nika Kirumase, leader of the Asterions-a splinter group of former Anadens thought aeons dead-arrives bearing a warning of a terrifying enemy advancing across the void. Known as the Rasu, the powerful race of shapeshifting metal has already killed tens of thousands of Asterions in its quest to control all of known space. Nika's people have struck a blow against the Rasu, and now they race against time to prepare for the coming reprisal.
An alliance with humanity stands to give them a fighting chance against their enemy. But for humanity, such an alliance may cost them everything, pushing the fragile peace they fought so hard to achieve to the breaking point and beyond.*In Amaranthe, where exotic alien life, AIs, wormholes, indestructible starships and the promise of immortality rule the day, no feat seems out of reach for humanity.
But when the worlds of Aurora Rhapsody and Asterion Noir collide and the Rasu horde descends upon them both, more will be asked of heroes past and future. More will be given and more taken, and when the dust settles the very fabric of Amaranthe will be changed forever.