Long ago there was a species known as the Precursors and once they became a space-faring race and saw that they were the only species in the galaxy, they felt very lonely. So they decided to create life to inhabit the planets that they found. They created specific habitats for each creature that they created and ruled over all their creations and they accepted the Precursors rule but they would sometimes annihilate races that they deemed unworthy but despite this, everything was fine. Until, one of their members grew power-hungry and wanted to take everything for himself and started a rebellion and it was during this rebellion that the Flood was created. However, he was defeated and as punishment they changed his name to the Fallen One and instead of killing him, they decided to exile him in hopes that he would one day repent but his time in exile only made him more evil and hateful of his brethren and plotted revenge. Now, he had always thought that all races were inferior to him and had no regard for life but he saw potential in a then-primitive race called the Forerunners to help him get his revenge. He knew that because the Precursors had targeted the Forerunners as another race to be erased. He told the Forerunners of the Precursors' plan and taught them how to use Precursor technology and when the Precursors came, the Forerunners were ready and wiped out all of them but a few. The surviving Precursors fled the galaxy and the Fallen One's revenge against his brethren was fulfilled and the galaxy was his to do with. But the Precursors had created a species known has humanity as a way to counter the Fallen One and were able to command the Fallen One's Flood to test both humanity and the Forerunners. The Fallen One, having learnt of this, turned the Precursors' benevolent plan of testing both races into another evil asset of his and released them at the right time on humanity. Humanity fought back against the flood but also expanded onto Forerunner territory and was forced into a battle between both species while the Fallen One watched all this unfold. Around this time, after the defeat of humanity and their devolvement, the Fallen One ordered the Flood to hide outside the galaxy and wait. 10,000 years later, they returned to attack the galaxy once again and the Forerunners were caught off guard. Years into the Forerunner-Flood war, he confronted the Didact and the Librarian husband and wife who he determined both to be a threat to his plans and attempted to destroy them but in a last ditch effort, the Didact was able to seal the Fallen One's spirit and geas inside of him. Inside of the Didact, the Fallen One was unable to completely control the Didact but could slightly influence his decisions leading to the Didact using the Composer to create Prometheans to fight the Flood. The Fallen One was defeated once again when the Librarian sealed the Didact in a cryptum on Requiem with all of its Prometheans and during their absence the Flood were wiped out by the Halo Array. Now, the Didact with the Fallen One inside of him, has remained imprisoned in Requiem until now.