Mendicant Bias

 The story of Mendicant Bias was told through forerunner terminals found in the campaign levels of Halo 3. 

Creation and Rampancy

 Mendicant Bias was created by the Forerunners with the hope that it could study the first Gravemind, and  learn it's weaknesses. But the Gravemind had other plans, it contacted Mendicant Bias and convinced him that the only way sentiment life could "progress" was to evolve into "superior beings". These superior beings were to take the form of the Flood and AI like Mendicant Bias. It also told him that the forerunners "Mantel"  had doomed the galaxy. Mendicant Bias eventually began to believe the Gravemind and hate the forerunners. At this point it became Rampant.
 

The fall of a Titan

Mendicant Bias was able to lead a Flood assault on the Forerunners defenses. The Forerunners were caught off guard by Mendicant Bias's betrayal and were not able to hold their line of defense. This forced them to abandon the Shield Worlds and accelerate their secret weapons, the Halo arrays. They also created another AI with the task of slowing down or destroying Mendicant Bias, called, Offensive Bias.  Mendicant Bias did not attack the Halo arrays, maybe because the Forerunners did not tell him their locations. But he did know about the Ark, and that the only way to go there was the portal on earth which would only activate with a Key Ship. The Forerunners were aware off his plan and began to destroy the Key Ships. Mendicant Bias then launched an attack on one of the remaining key ships that Offensive was defending. The battle did not go well for Mendicant though, ending with the destruction of his fleet and the Halo arrays being activated.  Offensive then broke the compound mind of Mendicant into fragments, and scattered them throughout space. Offensive took one of the pieces to the Ark.