The Perseus Arm is one of the major spiral arms of the Milky Way Galaxy]. It is home to The Royal Allegiance. Before about 100,000 years ago, the Perseus Arm, along with the rest of the galaxy, was populated by the Forerunners. When the war with the Flood became a foregone conclusion, the Forerunners activated the rings, starving the Flood of biomatter but destroying all sentient life in the Galaxy.
About 25,000 years later, when the Flood had vanished, automatic systems activated, sending humans back to Earth from the Ark on three Forerunner ships- one Keyship and two other ships- on auto-pilot. Each of the ships carried about 7,000 humans, kept unconcious and restrained by forcefields. Unfortunately, just as the ships entered the galaxy in slipspace, a slipspace anomaly caused them to drop out of slipspace. As they passed a star after entering the galaxy, two of them were hit by a solar flare, causing them to malfunction. It can be assumed that the slipspace anomaly was caused by the solar flare, but there is no evidence to support this. The single Keyship them vented its human cargo into space, killing almost one third of the galaxy's human population. This later drifted into Covenant space, where it was found by the Prophets (San 'Shyuum). They worshipped it and incorporated it into their religion.
The second ship hit by the flare contained a genetically enhanced race of humans, The Order. Their purpose was to watch over the humans of the other two ships once they arrived at Earth, The ship began firing its thrusters in random directions, until it reached an inhabitable planet, colonised 25,000 years earlier by the now-extinct Forerunners. Emergency protocol initiated, and the ship safely crash-landed and released its passengers, who would utilise the Forerunner technology to advance. This human population would later grow to become the Royal Allegiance. Their arrival on the ship became mythology.
The third and final Keyship arrived at Earth and similarly release the human cargo. It is unknown where the ship came down, but the human population that eventually arrived at Earth, 25,000 years after the Forerunner's death, was much smaller than they would have wanted (see human population bottleneck theory).