|Warning: This article may contain plot spoilers.|
The Prophet of Regret is one of the three Hierarchs in charge of the High Council in the Covenant. Like all Prophets, the Prophet of Mercy rides in a hover chair and is physically frail.
The Prophet of Regret plays a relatively large role in the backstory of Halo 2, as he leads the attack on Earth at the beginning of the game. His flotilla of Covenant warships jumps into Earth's space and begin an attack, which is how you begin the game.
However, the Prophet of Regret not only appears in cutscenes of Halo 2, but also makes an appearance in-game in the mission "Regret", specifically in the sub-mission titled Sorry, Were You In The Middle Of Something?, where the Master Chief confronts him and his horde of Honour Guards. In that mission, the player is exposed to the characteristics of the Prophet hover chair, which allow the Prophets to hover and fly around quickly. The chair also boasts upgraded shields which are impenetrable, and wields a golden beam like weapon similar in effect and power to a Fuel Rod.