From the Halo 2 Manual (Human Version): Little intel is available on these creatures. We know that they are administrative or religious in nature and have little to do with actual battlefield circumstances. None has ever been killed or captured, and they appear to be few in number.
From the Halo 2 Manual (Covenant Version): Our guiding light and the directors of our purpose. They are father to the children of our Covenant, and they will lead us to truth and to perfection. The Prophets of Truth, Mercy, and Regret will navigate us through the task that awaits us.
The Prophets are the leading class of the Covenant society. The lower classes do whatever the Prophets say, because the Prophets are able to communicate with "the gods." The Prophets ride on a hovering platform, probably propelled by the same type of systems as the Covenant Ghost. Little is known about the Prophets to date, and more information will be added as it becomes available.
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