- “Halo! Its divine wind will rush through the stars, propelling all who are worthy upon the path to salvation.”— Prophet of Mercy
The Prophet of Mercy was elected to the High Council before either of his fellow Hierarchs, the Prophets of Truth and Regret, were even born. His tenure was marked by several tense debates with Sangheili High Councilors over the proper dispensation of Forerunner artifacts. During that time, he was considered one of the last of the Old Guard, as he publicly questioned the wisdom of the San'Shyuum's demilitarization at the close of the Sangheili-San'Shyuum war. This brand of thinking may have contributed to his later willingness to help replace the Sangheili with the Jiralhanae.
23rd Age of Doubt
By 2525, he had apparently given up politics in order to more effectively indulge his spiritual side. He served as the Philologist on the Forerunner dreadnought, where he led the priests prior to his ascension to Hierarch. He was present when the Minister of Fortitude and the Vice Minister of Tranquility came to ask the Oracle of the Dreadnought to give its blessing to their intent to become the next Prophet Hierarchs. In response, the Oracle attempted to launch the Dreadnought in order to transport the human population of Harvest to the Ark, during which time it referred to the humans as its creators. The Philologist, overcome with spiritual awe, beseeched the Oracle to complete its task; however, when the Dreadnought's electrical pathways were short-circuited by the Lekgolo worms that had been moving within them, thus stopping the Dreadnought's launch, the Philologist regained enough sense to disconnect the Oracle from the Dreadnought's systems.
Because the Philologist had heard the Oracle's secrets, which would shatter the Covenant if they became known (it was thought that the Forerunners had transcended when they activated the Halo Array; if the humans had been left behind, that meant that not all who walked the divine path would necessarily gain the same transcendence upon the coming of the Great Journey), the other two Prophets offered for him to be their third Hierarch. The Philologist agreed, if perhaps reluctantly; he was later seen picking irritably at his new robes during the Hierarch initiation ceremony. With his help (doubtlessly his former career as a member of the High Council was very helpful for their campaign), the three San'Shyuum successfully rose to the mantle of Hierarch. The Philologist chose the title of Prophet of Mercy.
Trial of Thel 'Vadamee
After the Destruction of Alpha Halo, the Commanding Sangheili who was responsible for the loss of the ring, was brought before Mercy, Truth, Regret (attending as a hologram) and the High Council to be trialed for his failure. Upon confirming there was only a single ship fleeing Reach and its glassing, Mercy angrily questioned why it wasn't destroyed with the rest of the human ships. As the council created an uproar, Mercy attempted to calm down the Council. Mercy would come to witness Truth declare Thel a Heretic as he was escorted away, by Tartarus and his Jiralhanae.
Later, an obviously displeased Tartarus was ordered to bring Thel in front of Truth and Mercy. Initially Tartarus had thought the disgraced Sangheili was to be executed, however Truth and Mercy had other plans. As an effort to help Thel regain his lost honor, The two Prophets allow Thel to take on the armor of the Arbiter, particularly to quell Sesa 'Refumee's Heretics. He mentions a bit of Covenant history, citing the Unggoy Rebellion and the Taming of the Lekgolo as cases where an Arbiter was appointed to resolve the conflict, stating the Arbiter position was of huge importance to the Covenant itself. Responding to Thel's mentioning of the Councils wanting him dead, Mercy mentions the tasks that were to be undertaken by the Arbiter were to be suicidal, and he would die as all others had before him.
While Regret's attack on Earth had failed and resulted in him being chased by the UNSC to Installation 05, the Prophet had sent a message to Truth apologizing for his brash actions. Truth proceeded to reprimand Regret, claiming it was thanks to Mercy that he did not publically condemn Regret for his foolish actions.
After the death of the Prophet of Regret by John-117's hands, Mercy alongside Truth had given the Arbiter who had returned successfully from his last mission, a new assignment. Mercy stated that while feeling satisfied with the continued killing of humans, they had felt sorrow after the destruction of one of the rings. He would reveal that the Prophets had questioned the captured 343 Guilty Spark about the Halos; and became aware of the Activation Index required to activate them. With this information Truth and Mercy assigned the Arbiter to a new mission; obtain the Sacred Icon at any cost. Tartarus was also ordered to assist in the capture of the Sacred Icon; however he was secretly ordered to kill the Arbiter, and acquire the Index himself.
Following the Beginning of the Great Schism, Mercy seemed to be more of an adviser or supporter of Truth than an equal, and still supported Truth's actions, even as it seemed that Truth was unraveling the very foundations of the Covenant. This indicates that Mercy was fully involved in Truth's machinations from the beginning, something that even Regret failed to completely achieve.
Great Schism and Death
As he watched Truth deliver a speech upon the successful acquisition of the Sacred Icon, John-117 was teleported by The Gravemind directly into the Council Chambers. Upon noticing John, Truth, Mercy, and a handful of Jiralhanae Honor Guards fled. Later, as the two High Prophets were about to depart for Earth with the Forerunner dreadnought to begin the Great Journey, a swarm of Flood Infection Forms attacked them and the Honor Guard Jiralhanae.
A single Infection Form managed to evade the guards, latch onto Mercy's neck, and begin devouring his windpipe. Tartarus attempted to save him, though Truth intervened and let Mercy die in order to become the sole leader of the Covenant, saying, "The Great Journey waits for no one, brother. Not even you." Before his death, he was found by John-117, who questioned where Truth was going. He answered, "Earth, to finish what was started. And this time, none of you will be left behind." 
John-117 then violently tore the Infection Form from Mercy's neck, killing the old Prophet instantly but saving him from being infected by the Flood. However, Mercy was mentioned by the Gravemind to be a part of him.
Mercy can be seen as passionate, wise, and knowledgeable, as seen in the cutscene before the level Sacred Icon where he shows his excitement about Halo, expressing hope that the Great Journey should begin very soon. It's very possible that he has a sense of propriety, seeing that he talked Truth out of condemning Regret, as described on the level Regret, but that doesn't necessarily mean that he is merciful. He is perceptible and an authority on the Forerunners and their knowledge, he can be considered the Prophet of Truth's right-hand. Without Mercy to advise him later, Truth becomes much more irrational and aggressive in Halo 3, even toward his subordinates (very much like Regret was before the Great Schism), although it is just as likely that Truth is showing his true personality after years of cool political veiling.
- Like all the other Hierarchs, Mercy's name is ironic. He is given no mercy by the Prophet of Truth and he showed no mercy either to the humans or to Thel 'Vadamee, whom he wanted dead.
- In Halo 2, Mercy was voiced by the late Hamilton Camp.
Halo 2: Anniversary
- Halo Waypoint - Prophet of Mercy
- Halo 2 Prophet of Mercy Exclusive
- Halo: Contact Harvest, page 271
- Halo: Contact Harvest, page 275
- Halo 2, campaign level, The Heretic
- Halo 2, campaign level, The Arbiter
- Halo 2, campaign level, Regret
- Halo 2, campaign level, Sacred Icon
- Halo 2, campaign level, Quarantine Zone
- Halo 2, campaign level, Gravemind (level)
- Halo 2, campaign level, High Charity (level)
- Internet Movie Database - Hamilton Camp