Echo Rift (Riven Worlds Book Three). Amaranthe, #16

"Everything we have done from the start of this endeavor has been risky. Is the goal not worth any risk?"In the aftermath of Namino's liberation, everyone seeks a moment of peace to catch their breath and regroup. But the Rasu will not be waylaid for long, and peace soon becomes a race against time before the next onslaught. Alex and Caleb search distant stars and the ruins of a dead civilization for a secret weapon that may hold the key to defeating an enemy who cannot be killed by conventional means.
In the Dominion, Nika and Dashiel probe the mysteries of kyoseil in search of answers, but the Asterions' bond with this ancient and enigmatic life proves more complicated than they ever realized. "You act as if there are no consequences for your actions and no price for your foolish decisions. It is time to remind you that there is always a price to be paid."Formidable as they are, the Rasu are not the only foe seeking to destroy Concord.
The Savrakaths have seen their cities leveled and fleets decimated by rebel Anaden forces. Now they marshal their last resources for a final vengeance-fueled strike-against the wrong enemy. After a failed coup attempt, the Anaden leadership bickers and squabbles amongst themselves-until a ghost from the past returns, determined to finally unite his people. But those who have wielded unbridled power for millennia will not give it up so easily. Yet amid the deadly battles and insurrections, struggles of faith, hope, friendship and love light the way forward, because winning is about more than just surviving.*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.