From Halopedia, the Halo wiki
The First Prophet had fought against the Didact's forces. After the San'Shyuum's defeat, he and the Didact established a bond with one another and for a time worked together. The First Prophet knew of humanity's cure against the Flood. Before entering a state of exile, he told the Didact that he would reveal this secret if the Flood ever returned, on the condition that the San'Shyuum be granted freedom from their quarantined system.
Following his revival late into the Forerunner-Flood war, the Ur-Didact traveled to Janjur Qom, seeking out the First Prophet. Arriving in-system after the San'Shyuum attempted an uprise against Forerunner defenses, the Didact was informed by captured San'Shyuum elders that the First Prophet had been killed during the initial attack when his palace was destroyed.
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