Judgement Day (Ark Royal, #20)

For uncounted millennia, the twin Dyson Spheres - one intact, and host to a population unaware of their origins or even the nature of the world around them; one seemingly little more than debris orbiting a dying star - have remained a mystery, their seemingly all-powerful builders as enigmatic as the supertechnology used to construct and maintain the humungous megastructures. But now, in the wake of HMS Endeavour's discovery of the spheres and the arrival of a multinational fleet to explore - and exploit - the alien tech, everything has changed.

The Builders have returned.

And for all of humanity, and every other known race, it is nothing less than Judgement Day.