This article is too damned short, Marine! You will help Gruntipedia by so it will fit the screen, fool!
The Prophet of Mercy was an old prophet who was literally 220 years old (Even the liars at Halopedia state this).
Mercy had a problematic upbringing, with a mother spending all her money on booze and a father addicted to kitten-huffing. He was eventually placed in psycological ward when he almost died of laughter because of a dead Grunt. Fighting his mental issues and balancing on the brink of lunacy, he was released and elevated to the rank of High Prophet and was celebrated in a big party. His mental retardation made him believe he had gotten a job as a cook on Burger King and proceeded to take every attendants order.
He proved himself mentally challenged when he thought Installation 05 would cleanse the galaxy of infidels with a holy wind*. After ranting endlessly, he was eventually killed by AIDS. Looks like those AIDS didn't have mercy. Master Chief asked him where the hell the Covenant was going. Mercy proved that he was mentally retarded one last time and said that they were going to planet Noob. Then Master Chief shot him in the face, screaming, "Take that, ugly!"
When ProFag was 360 noscoped by master chef, the phaggt expolded with cum and Mountain Dew with doritos DNA.
*It should be noted that the words Installation 05 and Chuck Norris sounds very much alike in the Covenant language.