As far as I know, the Mouth of Sauron was the messenger of Sauron himself AND the lieutenent of the Dark Tower (Barad Dur). He is not slayed in the books, although he is in the movie by Aragorn. In the books, he refers to himself as an ambassador who may not be attacked.
This whole scene happens at the Black Gate when Aragorn leads the host of the west into Mordor to bide time for Frodo and Sam to destroy the Ring.
Hope this helps!
