The Siege of Cask took place after the Brood War. The Zerg Swarm laid siege to the colonial outpost for fourteen months, eventually evolving the baneling to overcome and destroy the defenses. The Terran Dominion was aware of Cask's plight but refused to intervene until the very end; the battle was used to observe zerg evolution and colonial resourcefulness.
The zerg began the invasion of Sorona by attacking the outlying settlements. The colonists retreated to Cask and abandoned the surrounding area. Cask was situated at the end of a high-walled canyon, where a thick arch of rock provided a natural roof. The location was proof against all but the heaviest air attack. The only ground route into the outpost was the Wedge, a bottleneck just wide enough for a big ore loader to scrape through.
Makeshift walls were erected at the Wedge and manned by shotgun-wielding miners. This was augmented by mounting mining lasers on the walls as well. These defenses held off daily zerg attacks. The terrans hoped the zerg would be unable to sustain the rate of attrition from a practically lifeless world. This seemed to be confirmed by the change in the attacking force's composition. Early attacks included more potent strains, such as devourers, hydralisks and ultralisks. As the months wore on the zerg attacked only with smaller strains like zerglings.
Seven months into the siege, Cask's doctor committed suicide. By then supplies had run low and malnutrition set in. Hydroponic gardens and a native mold prevented starvation. The mold provided protein and carboxylic acid but its acidity caused dental problems.
At some point the zerg began constructing spore cannons in the surrounding mountain ranges. These were not turned on the colony even when completed.
The Marines Arrive
After eight months of siege, terran reinforcements arrived in the form of the 128th Platoon, Dominion Marine Corps, commanded by Lieutenant Travis Orran. The 60-strong unit was en route to Anselm aboard a Minotaur-class battlecruiser when the ship was recalled to Korhal IV. The platoon was deposited at the deserted starport 15 miles from Cask without being briefed on the local situation. They set out to obtain quarters at Cask.
Five miles from the colony the marines were ambushed by burrowed zerglings. Orran quickly rallied his men. Twelve marines were killed during the attack, including Private Braden, and three more died afterward from wounds. Only ten zerglings were found dead. The platoon readied themselves for more action as evening set in, although it seemed unlikely they would survive the night.
The colonists heard the battle and sent a big ore loader to investigate. The marines embarked and carried back to Cask. Although there was disappointment that the 128th was not part of a large relief force the colonists made the marines welcome. The colonists supplied the soldiers with ammunition from a modified tool factory and water. The marines' supplies helped lessen shortages, and the platoon's medic, Private Maren Ayers, began addressing the medical treatment backlog. "Tech-savvy" Private Hughes confirmed the colony's communications equipment was functioning properly.
By the time the marines arrived, eight mining lasers were operational at the Wedge and the gate was made of paristeel.
Method Behind the Madness
The zerg attacks took on a familiar pattern. There were daily attacks at indeterminate times, but no more than one per 24 hour period. It started with a dozen zerglings and escalated to a wave of hundreds. Nonetheless they barely managed to reach the walls. After each attack the corpses were burned away by the mining lasers.
Private Ayers perceived the methodical zerg approach and believed the attacks were part of a zerg evolutionary program, on the basis of her father's theory that reducing the size of a population increases the rate of mutations. Without hard evidence she could not convince Lieutenant Orran to strike out and destroy the hive responsible. Orran was unwilling to risk the platoon in the midst of a prolonged siege.
In search of proof, Ayers fashioned a primitive field laboratory from her combat suit and used it to analyze tissue samples taken from dead zerglings. The tissue had green bulbous pustules containing material partially based on the mold the terrans were eating. The mold was identifiable by its peppery smell. Six months after arriving on Sorona, the medic attempted to dissect a sample the size of a fingernail with a med-laser. The resulting explosion destroyed her room. Ayers was wounded but survived by being suited up. Nonetheless she was unconscious for two hours and the suit was badly damaged.
The zerg attacked the morning after Ayers' accident, but this was followed by a pause. After a week of no attacks, Privates Evans, Godard, and one other marine volunteered to scout the surroundings. They returned to report they had found a valley of zerglings mutating into banelings. The terrans believed the zerg were dying from disease. Only Private Ayers realized the situation as it was. The banelings attacked as she used Cask's communications gear to appraise the Dominion of the recent development. Word of the new mutation galvanized the Dominion to action; a battlecruiser fleet was dispatched to Sorona and arrived four days later.
It was four days too late. The defensive walls were gradually destroyed by the explosive banelings. The banelings themselves evolved to roll into balls for faster movement. Eventually the tide wiped out the colonists and marines. The zerg withdrew after destroying Cask. Twenty-four hours before the fleet arrived the zerg began disseminating the new strain to other worlds with the spore cannons.
Lieutenant Orran and Private Ayers survived by hiding in the cliff face and were subsequently rescued. Ayers was debriefed by Captain Serl Gentry. The medic accused the Dominion of callously abandoning Cask and ended the interrogation explosively with a smuggled baneling pustule. An audio record of the session was later recovered from the wreck of the battlecruiser Emperor's Fury.