The Illusive Man's plan to capture Omega was elegantly wrought. Experimental specimens called adjutants "escaped" from a Cerberus facility beyond the Omega-4 mass relay, took command of a Cerberus convoy, and arrived on Omega to wreak havoc. With more adjutants incoming, Cerberus reached out to Aria T'Loak, claiming it needed her help to stop its wayward test subjects. Reluctantly offering her own forces in an alliance, Aria joined Cerberus's General Oleg Petrovsky aboard his ship to oversee the battle. When the adjutants overran the Cerberus-Omega fleet, Petrovsky and Aria escaped through the Omega-4 relay.
However, Petrovsky's subordinate Colonel Raymond Ashe had remained behind on Omega to take control of the station's defenses while Cerberus reinforcements approached. Announcing that it was protecting the galaxy from the adjutant threat, Cerberus seized Omega, giving the Illusive Man full control of the Omega-4 relay and the space beyond. Without Aria to keep them in line, the various criminal and mercenary factions on the station erupted, fighting both Cerberus ground troops and each other.
Meanwhile, Aria was taken captive by General Petrovsky. Realizing that the adjutants had simply been a ruse to capture Omega, Aria managed to escape back to Omega, slipping away into its underbelly to organize a resistance force.
Eventually, Petrovsky ordered the Cerberus forces to evacuate the station, but this was yet another ruse -- this time to lure Aria out of hiding. With his troops out of harm's way, Petrovsky summoned a massive Cerberus fleet and presented Aria with an ultimatum: leave Omega or lose it. Petrovsky threatened to destroy the station rather than lose control, and with no alternative, Aria left, vowing she would return to claim Omega and take her revenge.