Independence: Book 4 of The Legacy Fleet Series

We hoped, desperately, that they came in peace.
We were wrong.
Thirty years after the Second Swarm War devastated Earth and its colonies, a powerful, mysterious alien ship has invaded our space. Entire planets are ravaged, whole moons shattered. Any starship sent against it never comes home.
But Admiral Proctor, a war hero from our last brush with annihilation, is called out of retirement to take the reins of humanity's newest starship. The ISS Independence and her crew, with Admiral Proctor at the helm, will stand as Earth's last defense. Somehow, against all odds, they will save us, even when our enemy is not just an unstoppable alien ship, but a ghost from humanity's past bent on its utter destruction.
And if they fail, we fall.
For good.