The High Prophet of Mercy was one of the three Prophet hierarchs who led the Covenant Empire (the others being the Prophet of Truth and the Prophet of Regret). Mercy, along with Truth and Regret, is one of the main antagonists of the Halo series. He was the eldest of the three and was originally a Philologist and a Councilor.
When the Great Schism, Mercy along with Truth and the Brutes made their way to the Phantoms on High Charity. The Flood suddenly attacked them and one attacked Mercy, Tartarus was ordered by Truth to not save Mercy because he wanted to become the ruler of the entire Covenant Empire. After following the order and abandoning Mercy, the Prophet was killed by the Flood Infection form.