The planet of Endeavor was a very earthlike planet with only a slightly higher gravity. The military had claimed the planet because it was far enough away from any other inhabited planet that nobody was likely to stumble across it making the perfect place to perform top secret research. Discovery city housed the support staff and military personal that worked on and/or for the Endeavor military research base which the planet was named after. They had built the base and the city while terra-formers made only minor alterations to make the planet feel even more like earth.
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The technician wiped the sweat from his brow as he slowly increased the power to the device, thinking he didn’t care what the professor said jump capacitors were not designed to be charged in gravity well. One more turn and the poor man would never know how right he was. The military wanted a new bomb and Professor Hoffman was determined to be the one to give it to them. Unfortunately he lacked a complete understanding of the device he was basing his bomb on. All he knew and cared about was that jump capacitors were designed to store a huge amount of energy, overcharge it and like any other capacitor it would explode. The problem was that jump capacitors not only stored the energy it focused it for one purpose, breaching the dimensional barrier.
The technician turned the dial one last time increasing the charge going to the miniaturized capacitor and it detonated. The bunker Professor Hoffman had claimed would contain the blast vaporized in an instant. The technician just started to look at the new readings when he and everyone else in the facility a half mile away from the bunker were ripped apart on an atomic level as the energy tore a hole in the barrier between our dimension and dimension where ships went to travel light years in days.
The black lifeless void that is jump space now had a light source and the creatures that inhabit that void took notice. In a time before remembering something or someone had created the barrier between the dimensions to keep those creatures from reaching the dimension of man, now there was a hole and those creatures wanted to feast on the souls of humans. They swarmed the breach forgetting their natural hatred and distrust of each other too eager to finally reach the other side of the barrier.
Some creatures were close enough to where the rift happened that they were pulled across through the fragmented edges. Luckily only a few were close enough to appear in the dimension of man this way. Their murderous rampage was the only herald of the horde to come. Discovery city was doomed and its citizens either fled quickly or died horribly.