Gwaihir is the same eagle that saves Gandalf from Isengard in The Fellowship Of The Ring
Most of the plot of this game is not based on source material. Agandaur is not mentioned in any of Tolkien's books or otherwise. The story was crafted specifically for the game and was designed not to conflict with the book or movie canon.
The game's main story is set during the three Peter Jackson's movies.
Though the character models are replications, unlike previous games in the LOTR franchise, this does not feature the voice talent of any of the original actors from the films.
Agandaur is a black Numenorean. Of the same clan as the Mouth of Sauron seen in the extended edition of the Return of the King (and featured in the book).