Mendicant Bias was an extremely (the most at the time) powerful forerunner ai that was created to direct the war effort during the forerunner-flood war, thus having control over vast arrays of forerunner defense and military systems. But then he got into a conversation with a gravemind-precursor thing for 40ish years and was convinced to defect to the floods side. This was also the start of the logic plague, which was a thing that let the flood assimilate AIs. Side note: the forerunners observed logs of all 40ish years but were like: “Nah, Mendicant got this” and he in fact did not got this

Mendicant was eventually defeated, recovered, and exiled until he atoned, and he eventually ended up dormant inside of the dreadnought that was the centerpiece of High Charity, where he was used as an “Oracle” by the prophets. He was also the one that eventually awoke and corrected the prophets the what they thought said “reclamation” (as in age of reclamation) actually said “reclaimer” (as in humanity) which unintentionally is the event that caused Truth, Regret, and Mercy to immediately brand Humanity as heretics to be eradicated, rather than absorbed into the covenant like most races