Into The Labyrinth
Abarrach was a huge volcanic asteroid-like world. The Sartan and Mensch lived inside its honeycombed tunnels—designed to provide minerals and metals to the Kicksey-winsey—but they turned out to be loaded with poisonous volcanic fumes.
The mensch were wiped out swiftly, though the Dwarves hung on the longest, and the poisons taxed even the magic of the remaining Sartan to create breathable air. Huge columns called Colossi had been created by the Sartan to purify the air and keep the world warm in the long run, but they began to fail almost immediately (because they did not receive the necessary energy from Pryan).
With all the mensch dead and Sartan magic stretched to the limit, the Sartan of Abarrach turned to the forbidden arts of necromancy, using the corpses of the dead to supplement a lost workforce. While at the start it seemed a logical (if desperate) move that would help, it proved to be the most critical and tragic mistake of the whole Sartan race, as the dead eventually escaped from Sartan control.
But there was an even worse result the use of necromancy had, one far more disastrous for the Sartan: For every life brought back, every soul prevented from passing on, another life ends untimely. For each dead body resurrected, another member of the species, somewhere, dies. The continuous restoration of dead Sartan on Abarrach resulted in the mass deaths among the Sartan populations of other worlds.