The Devastation Ark

From the introduction:

Eons ago, two highly advanced civilizations—sivvs and kishalee—battled each other in seemingly endless conflict. As a last-ditch effort to end this cycle of destruction, the sivvs developed a superweapon called the Stellar Degenerator, which could vastly accelerate the rate of nuclear fusion in a star and drain away all of its energy, turning it into a cold, lightless, black dwarf star. However, in a daring gambit, the kishalee seized control of the superweapon before it could be launched and hid it away in a secret demiplane. The Sivv Dominion no longer had the means to keep pace with the war effort, and it was defeated. In just a few generations, sivv civilization ceased to exist.

Or did it?