Sorona is a rust-orange, nearly lifeless Terran Dominion mining world, populated by unique species such as an acidic mold. It is orbited by a slender asteroid ring. Close to the ice world of Anselm, Sorona is not considered a strategic point.
In 2503, the world was attacked by the Zerg Swarm. The colonists of Cask, a naturally well-defended outpost, came under daily attack. The colonists built "the Wedge", a barricade, in order to enhance the natural defenses. For eight months the colonists called the Dominion for help, but the Dominion, though aware of the calls, did not respond. The colonists were forced to subsist on a native acidic mold which, while nutritious, was bad for their dental hygiene. Meanwhile, the size of the attacking swarms decreased, and only minor breeds such as zerglings were being produced. The colonists seemed to be winning.
"Salvation" came in the form of the 128th Platoon, which had been deposited there so it's battlecruiser could quickly return to Korhal IV for upgrades. However, the platoon was not aware of the local situation, and quickly lost a quarter of their members to a zergling ambush.
The rest of the platoon were rescued by colonists and taken to Cask and contributed to daily battles against the zerg. The zerg continued to weaken while daily throwing themselves against the Wedge. Meanwhile, medic Private Maren Ayers studied the zergling corpses, and discovered they contained explosive nodules derived from the native acidic mold. The discovery landed Ayers in the infirmary and her lieutenant believed she had been attempting suicide.
The zerg attacks promptly stopped. Eventually the marines scouted the area, discovering seemingly sick zerglings transforming into banelings. Not recognizing the threat, the marines killed many and then left. Ayers realized what was happening and managed to contact the Dominion just as the banelings assaulted Cask. The zerg eventually destroyed the outpost and, in the four days it took for a Dominion fleet to arrive, used spore cannons to transmit information to other zerg-inhabited worlds.