The Covenant assaulted Cleveland after ColonelJames Ackerson told them about the Key of Osanalan, a supposed Forerunner artifact that was hidden in Cleveland, while under interrogation after his capture at the Battle of Mare Erythraeum. Ackerson said it was a necessary component for the firing sequence of the Halo array, in order to properly initiate it. Ackerson simply told the Covenant about it in order to save his brother and confuse the Covenant. The Covenant would not destroy the city before getting the Key, and Ruwan's knowledge of the Key would help him escape, as the Key was actually an object they made up when they were children.
The Covenant quickly assaulted Cleveland, taking light casualties as they battled Colonial Militia, Marines, and armed civilians. However, during the battle, the Covenant did not make any attempt to destroy the city or kill any civilians. After securing most of the city, the Covenant instead captured many civilians and brought them to a stadium in order to question them about the Key of Osanalan.
Just as the civilians arrived, ODSTs attacked the stadium and swiftly killed most of the Covenant forces. They then evacuated the civilians. Ruwan stated that he knew what the Covenant were looking for, and he and Tyla were brought to a secret UNSC base in the sewers of Cleveland to meet with the leader of the remaining UNSC forces in the area, Sergeant Twyker.
Although hostile at first, Twyker and Intel Operations Manager Wesley realized the importance of the Key. They contacted the Covenant to inform them that they had the key and to meet them for it. Ruwan volunteered to partake in their new mission and was injected with an organic tracking material.
He then was taken to Chagrin Falls, Ohio where a Brute Chieftain demanded he give them the key. Ruwan told them that he was the key. Confused, the Brutes brought him aboard the Harbinger of Piety before the Minister of Inquisition, who questioned him of Halo but Ruwan was confused. Inquisition then revealed he knew about the tracking material inside him and presumed that it was a homing beacon for rescue. Ruwan corrected him, stating it was a targeting vector for the MAC Platform Nassau to fire on the ship. Nassau Station then fired its cannon, destroying the ship and killing everyone aboard.
After hearing of the destruction of the Covenant ship, Ackerson proceeded to openly mock the Covenant, saying their pompous arrogance would cost them the war, before being beheaded by the Brute Ship Master, Lepidus.
Because of their gullibility, the Covenant lost many soldiers and ships, as well as an important minister, looking for a non-existent Key of Osanalan. However, this victory came at a cost; Cleveland was horribly damaged as a result of the furious urban combat that took place, and a large percentage of its civilian population was killed in the sudden invasion.