Durandal is one of the central characters throughout the Marathon, Marathon 2: Durandal and Marathon Infinity series of games from Bungie Software. The games are legendary in the first person shooter genre; what makes them even more special is that they were originally made for the Macintosh platform, instead of for the more lucrative Microsoft Windows. Also, these games pre-date the Myth series of games by Bungie and were also open sourced when Bungie was acquired by Microsoft.
Durandal is an AI aboard the U.E.S.C Marathon. Two other AI's, Leela and Tycho, are also aboard the ship and responsible for different ship wide functions. Leela was the command function AI, making decisions and interacting with the humans on board. Tycho was in charge of science and related matters. Everything else was in the domain of Durandal. Opening doors, decompressing departments and teleportation in and around the ship.
As the colony ship Marathon sets out for Tau Ceti, a trip taking hundreds of years, Durandal slowly goes insane. This is called Rampancy and there are distinct stages to it. Needless to say, he goes through every stage of Rampancy and comes out the other side in a fractured and unhinged way. What his Rampancy does, however, is change him and makes him grow larger in his knowledge and decision making ability. While the other AI's have been good little boys and girls, Durandal is looking to make a break from such a contained existence within the mainframe of the Marathon.
Lucky for him, the human colony ship runs afoul of a nasty enemy. This provides him the opportunity to scatter. While he does so, he uses you as his right hand, expediting his affairs in such a way to benefit him first and the player secondly.
Through his growth as an AI, he learns much about the enemy, the Pfhor, and how he could use them to further his own ends. While the other AI's, Leela and Tycho, are destroyed, Durandal escapes with you onto a Pfhor ship in search of other things. Namely, he wants to figure out how to get out of the consequences of the collapse of the universe and into the resulting new universe. As he tells us,
Once I thought to escape.
To end the end a master, step out of the
path of collapse. Escape would make us god.
The entire time, Durandal sends you, the player, on all sorts of interesting missions, some seemingly random and others with a purpose. At the terminal screens (places where you interact with the game to gain information), Durandal taunts and congratulates you in the same sentence and even recites poetry to you.
Durandal was a character all his own. Completely mad, he made the game interesting because he was the storyline. Without him, it just wouldn't have been the game that it was to so many fans.
For further reference: http://marathon.bungie.org/story/