He was one of the Black Númenóreans, and briefly appeared in person when he haggled with the Army of the West in front of the Black Gate (Morannon in Elvish), trying to convince Aragorn and Gandalf to give up and let Sauron win the war for Middle-earth. When Gandalf turned his proposal down, the Mouth of Sauron set all the armies of Mordor on to them.
Also known as the Lieutenant of Barad-dûr, he had served Sauron for much of his life, and had forgotten his own name. A man of great stature and also skilled in great sorcery, he was potentially the equal of other Dúnedain, but had fallen into darkness.
As the Mouth of Sauron, "...he entered the service of the Dark Tower when it first rose again" — there is a dispute over the exact length this implies. If it refers to when Sauron most recently returned to Mordor, it implies that by the time of his encounter with Aragorn and Gandalf the Mouth of Sauron had been in the service of his master for 68 years. It is possible, however, that because Sauron began the reconstruction of the Tower before actually returning to it, this might be the time involved. It has been theorized that the Mouth of Sauron possesses one of the dwarf rings, which would explain his age, power and deformation. This is not confirmed by any of Tolkien's writings.
One inconsistency about the Mouth of Sauron's title is that Aragorn states earlier in the second volume, The Two Towers, that Sauron does not "use his right name, nor permit it to be spelt or spoken". It has been conjectured by Christopher Tolkien that this can be understood in terms of Aragorn's information being out of date, relating to Sauron's previously hidden identity.
In the 2001-2003 The Lord of the Rings film trilogy by Peter Jackson, the Mouth of Sauron does not appear in the theatrical cut of The Lord of the Rings: The Return of the King, but he does appear in the extended version, played by a virtually unrecognisable Bruce Spence, with the words "LAMMEN GORTHAUR" (Sindarin for "Voice of Sauron") in Cirth runes written on his helmet. His helmet covers his entire face except for his mouth, which is horribly diseased and disfigured by all the evil he has spoken. His mouth is also disproportionately large, creating an unsettling effect. Jackson conceived of this idea long after the footage had been shot, and asked his special effects team to create the effect digitally. After having their idea of digitally turning his mouth so it appeared vertical on his face rejected by Jackson, the designers came up with the idea of rendering it twice as large as the original.
The extended DVD cast commentary mentions that Jackson considered different depictions of the character, such as having Kate Winslet (who starred in Heavenly Creatures, another Jackson film) play the role, partially to emphasise the temptations Aragorn was facing.
Another departure from the text consists of Aragorn decapitating the Mouth of Sauron with Andúril after the Mouth taunts him and Gandalf by revealing Frodo's mithril vest, leaving the others to believe that he failed in their mission and was tortured to death. But after killing the Mouth of Sauron, Aragorn refuses to accept the emissary's words to be true, in contrast to the despair the Mouth's words inflicted on the others. In contrast, the literary Mouth of Sauron claims diplomatic immunity, saying "I am a herald and ambassador and may not be assailed!" Gandalf then remarks he has not been threatened.
Jackson said that he had cut the encounter from the theatrical version due to its lack of effect; he commented that in the book it was dramatic since the reader does not know Frodo Baggins' fate, but in the film the audience knows that both Frodo and Samwise Gamgee are alive. Once the Mouth is beheaded, his horse and body are not visible in the following shot as Sauron's armies of Orcs march through the Black Gate, even though Aragorn's sword is visibly stained with his blood.
He is also featured in EA Games' The Lord of the Rings: The Return of the King, as a boss the player must defeat in order to beat the level "The Black Gate". He is also a playable evil "Hero" in The Lord of the Rings: The Battle for Middle-earth II.