Durandal is an AI originally aboard the UESC Marathon. Throughout the majority of the Marathon Trilogy, Durandal communicates with player the most. This includes setting the story and provides objectives for the player to complete.
Durandal is the UESC Marathon's function-control AI, in charge of controlling doors, stairways, and other functions. He undergoes rampancy over the course of the first game, having been purposely brought to that state by Dr. Bernhard Strauss as an attempt to achieve "stable rampancy." By the time he found Lh'owon, seventeen years later, he is meta-stable and guides you through the whole game. Although he is very sarcastic, with a cruel sense of humor, and seems to despise humans, he is not predominantly evil, and is never seen to abuse the cyborg or other humans without some ulterior motive. For instance, while it was in fact Durandal that secretly detected and then intentionally made the Pfhor aware of humanity's presence in order to hijack a Pfhor space-folding FTL ship -knowing full well that they would likely destroy the human colony and the Marathon, killing or enslaving every human being in the solar system before he could gain control of a Pfhor spacecraft- he considered his actions to be in humanity's long-term interest because an ambush set up by him (even with Tau Ceti's humans as patsies) would be better than the Pfhor stumbling on humanity's home in the Sol system later on.
Durandal is the plot focus of all three games. Indeed, it could be argued that he, not the Cyborg, is the main character of the series.
Durandal was constructed by Bernhard Strauss and subsequently put aboard the UESC Marathon along with Leela and Tycho. After Strauss' experiment to find stable rampancy, Durandal slowly became rampant. He then found and contacted the Pfhor so he could steal their ship. As the Pfhor were eventually driven off by the chaos he and the marine created, he transferred himself to their ship and threw all the Pfhor out in the vacuum of space. Before he exited the Tau Ceti system, he quickly teleported the marine in a stasis chamber aboard his new vessel, the Sfierra, which he would rename the Boomer. When he left, a pfhor fleet came to tau ceti and destroyed the colony and the marathon.
Somewhere between his travels he visited the sol system and taught humanity warp-grade missiles.
He would search seventeen years for the S'pht homeworld, Lh'owon. He was interested in it because of the Jjaro, an ancient race with incredible technology. When he eventually found the planet he woke the marine and an army of BoB's, which the Pfhor had captured, from stasis. He sent them to research the sph't history and mythology as they were very religious toward the Jjaro. He hoped to find something with which he could escape the closure of the universe and become a god. And he also wanted to find a new weapon with which he could crush the Pfhor.
However Pfhor battle group 7 arrived and he had to fight them of in space. He defeated half of their Western Arm, but was eventually forced to crash on Y'loa, the second moon of Lh'owon. As the Marine fought off wave after wave of Pfhor, Durandal's network was being breached by Tycho. After a long struggle, he ordered the Marine to destroy his core, but it did not stop the Pfhor from downloading Durandal into a containment device. He was kept aboard the Khfiva for a month before he contacted the marine through a secret terminal on My Own Private Thermopylae, warning him about Thoth's obsession with balance.
He eventually captured the Khfiva when the S'pht'Kr arrived and named it the Rozinante.
However when the Trih Xeem was deployed, it released the W'rkncacnter. The Marine was then sent through alternate timelines by the Jjaro station, looking for a way to stop the chaos. He eventually had to merge Durandal's core chip with Thoth, creating a hybrid A.I. referring to itself as we instead of I. It would speak both in both styles of Thoth and Durandal.
It is implied that Durandal's personality was dominant in this hybrid. As he would return to Earth ten thousand years later just so they "wouldn't forget him".