Halo: Uprising Issue 3
From Halopedia, the Halo wiki
In a flashback, the "Key of Osanalan" is revealed to be a fictional object made up by Ruwan Ackerson, and his brother, James Ackerson when they were kids. Colonel Ackerson spoke of the "key" and divulged this false information in order to prevent the Covenant from destroying Cleveland, where Ruwan resided. In response, Covenant forces forcibly assembled nearly all of Cleveland's inhabitants into a stadium for questioning about the Key. Ruwan and Myra are saved by a team of Marines and escape the city in a Pelican, where they tell the Marines what the Key of Osanalan is.
Meanwhile, aboard the Forerunner Dreadnought, John-117, attempting to assassinate the Prophet of Truth, is caught by surprise by a Jackal and shot in the back. He falls many stories before landing in front of a procession of Brutes guarding the Prophet of Truth. While the Spartan manages to get several shots at Truth, the prophet is shielded by his gravity throne and makes his escape, as the Chief is left to fight dozens of Covenant. He kills them all and starts to find a way off the ship.