From Halopedia, the Halo wiki
The Prelate was an official in the Forerunner government. In approximately 97,745 BCE, the Prelate received an automatically collated report of an object approaching the planet Seaward, which later proved to be the first Flood invasion since the human-Forerunner wars. He reported this information to the Auditor.
"Prelate" is defined as a high-ranking member of the clergy. As such, the Prelate may have been a religious official or scholar of the Mantle. This is also supported by the fact he is described as wearing "clerical robes".
List of appearances