When the Dark Lord Sauron lost his bling-ring, he decided he didn't want to show his face in public anymore. Because of this, he needed to recruit someone to do errands for him. This is where the Mouth of Sauon comes in.
He was a hobo living on the streets of Minas Tirith until Saruman told him about the excellent employment oppurtunites in Mordor, the kingdom of all things bad. The Mouth went there and was interviewed on life in Minas Tirith. Because he slagged Minas Tirith off so much he immediately got the job, and was sent out to meet the Army of Men who had arrived at the Black Gate, Mordor's doorstep, to challenge Sauron. He displayed his prowess of Trolling at this point.
The Mouth ridiculed Aragorn, the future king of Gondor, and Gandalf by showing them little Frodo's underwear, to show he was dead. It never occured to any of them that Frodo might have simply got too hot near the fires of Mount Doom and decided to take it off to keep cool, as it was in reality.
This made Aragorn pretty angry, and despite his best efforts to calm him down, all the Mouth got for his trouble was a quick decapaitation. Gimli the Dwarf summed up the situation in one simple phrase: 'Well, that concludes the negotiations.'