High Prophets

Summary
The High Prophets are characters in Halo 2. The three High Prophets serve as the highest leaders of The Covenant High Council, being theocrats and known as "Hierarchs". The Hierarchs are thin, hunched creatures with large craniums that float on levitating chairs decorated with golden backs, equipped with energy shielding, and armed with Gravity Cannons; they wear crowns adorned with small holographic projections of Halo; they have the ability to teleport. Unfortunately, you'll only get to deal with one yourself, but they are all prior to death or unfortunate events befalling them amidst the storyline of Halo 2...

High Prophet of Truth
Little is known about the High Prophet of Truth, except that he is the final remaining top Hierarch at the end of Halo 2 who offered the former Elite Commander of Alpha Halo I-04 to become the Arbiter and ordered him to defeat the Heresy, a Rebel movement that regards Covenant teachings as false, and to retrieve the Sacred Icon/Index on Delta Halo I-05.

He spearheaded genocide of the Elites so that the Brutes would replace them in society, the beginning of a New Order and a Civil War.

When Gravemind teleported Master Chief to High Charity, the Prophet of Truth (who wanted him dead) betrayed the Prophet of Mercy who was wounded by a Flood Infection Form and escaped to his Forerunner flagship, but not before Master Chief follows him.

The Prophet of Truth is heading for The Ark on Earth to probably finish what they started.

High Prophet of Mercy
Supposed oldest of the three High Prophets, Mercy was first seen present at the trial of the Supreme Commander of The Fleet of Particular Justice, but decides to try something different than an execution for the shamed Elite. The High Prophet of Mercy agrees with The High Prophet of Truth in promoting the Elite character to the Arbiter, and then sending him on a mission to kill "the real Heretic". Upon completion of his mission, he plans to invade Earth a second time (after Regret's botched invasion, this time perhaps to secure The Ark, begin the Great Journey, and wipe out the Humans). Before he can succeed, he is attacked by The Flood on High Charity and infected. Seeing this, the High Prophet of Truth tells Tartarus, "Let him be!" and then to Mercy, "The Great Journey waits for no one brother... Not even you." The Prophet of Mercy tells the Master Chief where Truth is going and dies.

High Prophet of Regret
The High Prophet of Regret appears to be the youngest of the high prophets. Contrary to his name, he appears to be reckless and headstrong.

The High Prophet of Regret leads the intial, botched attack on Earth. He lands troops in New Mombasa, East African Protectorate, the only place the Covenant land on Earth. The invasion is foiled, due in part to the actions of the Master Chief, and he then flees back to Delta Halo. It is speculated that he was not at all aware of the importance of Earth to humanity and simply assumed it was another human world. This accounts for his fleet being far smaller and much less prepared than the fleet that destroyed Reach. His action does not win him a reprieve... the Master Chief pursues and eventually assassinates him.

The quality of being dead does not entirely hinder the Prophet of Regret, as he makes a final appearance later in the game (revived to a degree by the apparent Flood leader Gravemind) in which he argues with 2401 Penitent Tangent.

Lesser Prophets
There are also a number of lesser prophets such as Pity, Supposition, and Disdain that form the prophetic branch of the Council, along with the Elites. These prophets can vote and make decisions, but ultimately all matters of import are ceded to the discretion of the three High Prophets.