The Mouth of Sauron had served Sauron all his life, and had forgotten his own true name. A Man of great stature, he was potentially the equal of the Dúnedain, but had fallen into darkness. He had learned much sorcery during his time under Sauron, and knew many of the Dark Lord's plans.
As a Black Númenórean he probably came from the Haven of Umbar, and it was stated that "he entered the service of the Dark Tower when it first rose again". The Dark Tower rose again when rebuilding began in the year 2951 of the Third Age, which means he entered Sauron's service shortly after that. Umbar had been defeated by Gondor under "Thorongil" (Aragorn's name in his youth) in 2980, so the Mouth might have fled to Mordor then.
If this mention of the second arising of the Dark Tower is taken literally (see below), then the Mouth of Sauron had been in the service of his master for 68 years by the time the Third Age ended. If his service began as a youth who was subsequently cowed, bewitched and indoctrinated by his new master, then the Mouth of Sauron could quite feasibly have no memory of his birth name.
During the Council of Elrond, the Dwarves of Erebor spoke of a Man who had come to tell them of the power of Mordor and persuade them to join its forces. Though the Man's identity is unknown, it is possible that he was the Mouth of Sauron.
The Mouth of Sauron briefly haggled with the army of the west in front of the Morannon, trying to convince Aragorn and Gandalf to give up and let Sauron win the battle for Middle-earth. Though he came before Aragorn and his men as an ambassador, he used quite insolent speech when he dealt with them. He tried to intimidate the army into surrendering by showing them the mithril coat of Frodo Baggins to make them think that the Ringbearer had been captured. When Gandalf turned down his proposal, the Mouth of Sauron set all the armies of Barad-dûr upon them.
The Mouth of Sauron's fate remains unknown.Some claimed that he was killed in the Last Battle of Dagorlad or escaped Eastward or Southward to become a tyrant of the Derngorgor in one of the more reclusive regions of Sauron's former empire.According to some rumors he was murdered by his fellow diadochs in a conspiracy by the other Masters of Mordor, though legend claims he returned from the grave as an undead Wraith and continued to hunt Gondor and it´s allies into the Fourth Age.
The Mouth of Sauron's Age
It is possible to interpret the words 'arose again' to refer to the power of Sauron rather than the construction of his Tower. In this case, the rebuilding of the Barad-dûr in 2951 was actually its second arising. It first rose again some time after 3220 of the Second Age, which means that, according to this interpretation, the Mouth of Sauron might have been at least 3200 years old at the time of the War of the Ring. Even for a Númenórean this was an exceptional age matched only by the Ringwraiths, and he therefore may have been a Ring-bearer, who by some magic of Sauron had not become a wraith himself. He may have worn a lesser ring, and not one of the great Rings of Power. Alternatively, he could have been granted one of the Seven Rings of the Dwarves, several of which Sauron had recaptured.
Support for this extreme old age is found in the statement that "his name is remembered in no tale, for he himself had forgotten it". Even Gollum still remembered his old name after 500 years.On the other hand it is explicitly stated that the mouth was not a wraith, but a living man.
the Mouth of Sauron in MERP:
- MERPLords of Middle-earth Vol II:The Mannish Races