Elder Worldship

Back in their spacefaring days, the Elders had an empire so massive that no one world could support their population for long. Even scattered across the universe, their cost of living, not to mention the cost of war, became so great that a system had to be devised to ensure their continued survival. The solution: Worldships.
An Elder Worldship is designed to support a civilization during the long journey between stars. Inside is contained enough square footage to put your average Habitable planet to shame, which in turn contains enough farms, housing, and entertainment to support billions of individuals for hundreds of years.
But, once those hundred years are up, issues begin to pop up. See, Elders never really mastered renewable energy on such a massive scale; after a thousand years, the air aboard slowly begins to turn, becoming choked with sour, acrid smog. The farms begin to wither, individuals begin to get sick.
However, the builders thought ahead. Every-thousand-years or so, the ship needs to make a pit-stop at some uninhabited world. The Worldship docks, then, wonder of wonders, pushes the planet into the goldilocks zone. Terraforming begins; all the smog choking the Worldship is released into the atmosphere in order to feed the growth of strange, new plantlife. A new ecosystem is built from the ground-up, complete with herbivores to keep the plant population in check, and predators to check them in turn. For another thousand years, this planet is precisely engineered into a paradise; the offal of which is exactly to sort of fuel the Elders need for another foray into deep space.
But soon enough, the planet achieves a balance, and stops producing the needed materials. The planet is now beautiful, but useless to them. So, they fire up the engines; vents of flame so powerful they instantly torch everything within a hundred-mile radius, kicking ash and dust into the atmosphere and causing a global ecological disaster; like a broom to their footprints. Their launch, more than likely, pushes the world back out of the goldilocks zone, leaving no chance for life to thrive on that dust-ridden ball ever again. Of course, none of this is intentional, but a natural byproduct of launching such a massive vessel from the surface of the planet. And after all, why fix it? They don't need anything from that planet anymore.

Price

Incomprehensible