Oberion the Great is a Forerunner army lord. He is the antagonist of the story
While the Flood had been an enemy for many Forerunners, Oberion had studied them with fascination. He had conducted a variety of experiments with them, which escalated to an obsession with him. When the rest of the Forerunner soldiers were trying to save their homes from the Flood, Oberion did not fight for the inhabited Forerunner homes.
When his flagship, the Cessation, was at the planet Anduu, a rampant 032 Mendicant Bias slipped in and released a portion of the Flood. The outbreak was contained, but a number of Flood forms escaped onto the surface of Anduu and infected the inhabitants.
"I am sorry, Cephaus. But we must leave them behind. There is no time to save the others, that wretched construct has seen to it. We must leave now, while we still can..."
After sending a soldier to light the Halo rings to wipe out the Flood, Oberion activated an invention his scientists had created: a Time Crystal. While leaving the surviving Forerunners to be killed by the Halos' activation, Oberion and all those aboard the Cessation fled their time.
After coming out of time travel, Oberion was not pleased to hear that the Halo rings were deactivated, and the Flood still alive. He ran into the remains of a Covenant Loyalist group, led by Othrys, who pledged service to Oberion and gave him knowledge of what had happened during the Human-Covenant War.
"Your Prophets were correct, Chieftain. We will crush the humans, rebuild our path into the Great Journey, and take those who were faithful with us into the divine beyond!"
—Oberion, extract from Extinction
Oberion easily won Othrys' loyalty with the same lies the Prophets had given to the Covenant, and began rebuilding the glassed remains of Eridanus II and Reach. 0 Shameful Anomaly, the secret unofficial Monitor of the Ark, had been present at Eridanus II as well.
When the Prowler Dusk arrived accidentally at Eridanus II, Oberion took Othrys' word that those aboard were weak and weren't to be feared. The Forerunner lord, confident the humans would quickly be eliminated, sent his forces to kill them, but failed twice. He had trusted Cephaus, one of his best veterans, to do so, but the Forerunner warrior betrayed him, and helped the humans instead. Eventually, as Oberion realized that the Spartans were going to destroy Eridanus II, he abandoned the planet, releasing the Flood onto it to slow the humans down, and flew for Reach on board the Cessation.
After seeing the deaths of Commander Vilarius and Captain Rugari, and the destruction of Eridanus II, Oberion became afraid of the Spartans. He was extremely shifty and paranoid, and even his sister Maelia kept her distance from him.
Oberion's confidence resumed when he used the power of the Flood to increase his own combat capability. However, his personality was more aggressive and monstrous, releasing the Flood onto his own soldiers, becoming more irritable with Maelia, and even destroying Shameful Anomaly when he was extremely angry.
Oberion waited for the Spartans to enter his fortress and fight him. He used the advantages of his technology and the Flood's physical enhancements to severely injure John-117, and was about to kill him, when a miscalculation or overlooked error during the experiment turned him into a full Combat Form, rendering Oberion "dead". He was officially killed when the Master Chief stabbed a broken Spartan Sword into the infection form in his brain.