The Prometheans are a division of Forerunner military and the main antagonistic force in Halo 4, replacing the Covenant. They were led by The Didact. They were once Forerunners, and later humans, but during a conflict with The Flood, they were converted to digital intelligences by The Composer.
In July 2557, over four and a half years after the end of the Human-Covenant war, John-117 and Cortana came across Requiem while drifting in the UNSC ship, Forward Unto Dawn. They and a force Covenant from Jul Mdama's Covenant were forcibly pulled into Requiem and crashed on the surface. They then encountered and fought the Promethean constructs inhabiting the shield world. The Didact soon took control of the Prometheans after he was accidentally awakened by John-117.
Bent on preventing humanity's ascendance to the Mantle in the Forerunners' place, The Didact's Prometheans teamed up with Jul 'Mdama's Covenant and went to war with humanity, with the Didact attempted to use the Composer to wipe out humanity. However, Master Chief and Cortana were able stop the Didact and destroy both the Composer and his ship Mantle's Approach with a HAVOK tactical nuclear weapon.
However, the Promethean constructs continued to battle the UNSC forces and engaged them when the UNSC Infinity returned to Requiem in February 2558. They eventually fell under the control of Cortana and became footsoldiers in her faction, The Created.