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The Great One became infected by a type of psychoactive spore, which had also infected a number of terran scientists. These scientists willingly sacrificed themselves to the Great One in a process called "communion."
Sometimes later, the Great One engaged the Dominion Marine Corps Torch Seven platoon, which had been assigned to retrieve the scientists. Shrugging off their gunfire, it managed to inflict 33% casualties on the unit before they were forced to retreat into a ravine. The Great One's size prevented it from pursuing them, and by this stage, some of their Impaler rounds were beginning to penetrate its carapace. Roaring, it retreated back into the jungle, crushing a lingering marine along the way.
The Great One later returned, pursuing the remnants of the platoon and scientists. Some of the scientists began the process of "communion," and in regards to the process of tearing them to pieces, the ultralisk obliged. Torch Seven retreated to the research facility, devising a plan to kill it. They set up a vespene gas bomb, and paced it under a rockface with the hope of collapsing it on the ultralisks. Gerhardt van Rijn, former lead scientist and head of the cultists, met with the marines, stating they would have a ceremony for the Great One that night, and they just didn't understand their connection to the Great One.
The Great One later came that night to their ceremony, and started killing the scientists. Sergeant Norwood Doakes lured the ultralisk to the bomb and collapsed the ravine on him, but it got back up and killed Jouvert. It began killing indiscriminately, slaughtering both Torch Seven and the cultists, and knocked over Doakes. It moved in for the kill on the Sargent, but Doakes got off a lucky shot that penetrated its armor. It fell over, and the marines of the squad continued firing on it, with the cultists rushing to throw their bodies on their god. Doakes felt the ultralisk's dying thoughts as it passed away, and even after the Great One died he said that he could hear him.