Durandal is an AI originally aboard the UESC Marathon. Throughout the majority of the Marathon Trilogy, Durandal communicates with the player the most. This includes setting the story and providing 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 becomes rampant over the course of the first game, having been purposely brought to that state by Dr. Bernhard Strauss in an attempt to achieve "stable rampancy." By the time Durandal finds 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 player or other humans without some ulterior motive. For instance, it was Durandal that detected and then intentionally made the Pfhor aware of humanity's presence in order to hijack a Pfhor space-folding FTL ship. He did this knowing full well that they would likely kill or enslave every human in the Tau Ceti colony and aboard the Marathon.
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 Dr. Bernhard Strauss and subsequently placed 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 in order to steal their ship, the Sfierra. The Pfhor were eventually driven off by the S'pht rebellion Durandal and the player created, and he transferred himself to their ship. With the help of the S'pht, Durandal assumed control of the Pfhor ship. Before exiting the Tau Ceti system, he teleported the player to a stasis chamber aboard his ship, which he would rename to Boomer. After leaving the Tau Ceti system, a Pfhor fleet arrived and destroyed the colony and the Marathon.
Somewhere between his travels he visited the Sol System and taught the UESG how to construct warp capable fusion missiles.
Marathon 2: Durandal
With the player in stasis, Durandal spent seventeen years searching for the S'pht homeworld, Lh'owon. He believed finding Lh'owon would allow him to escape the universe and find a weapon for Humanity and the S'pht to defeat the Pfhor. When Durandal eventually found Lh'owon he awoke the player and the humans aboard his ship. These humans had been captured by the Pfhor from the Marathon and the colony on Tau Ceti. He sent the player and them to investigate the history and mythology of the S'pht, to ultimately find a way to contact the lost 11th S'pht clan.
While the player is searching for information about the lost 11th clan, the Western Arm of Pfhor Battle Group Seven arrives and attacks Durandal in space. He destroys half of Battle Group Seven, Western Arm but is eventually forced to crash on Y'loa, the second moon of Lh'owon. The player finds information about the 11th clan and returns to Durandal's ship and attempts to rescue his core, all while Durandal's network is under attack from Tycho. After a long struggle, he ordered the player to destroy his core, but it did not stop Tycho from downloading Durandal into a special containment device. Durandal was kept aboard the Khfiva for a month before he contacted the player through a secret terminal on My Own Private Thermopylae, warning him about the S'pht AI Thoth's obsession with balance.
The S'pht'Kr are contacted, Durandal escapes Tycho's confinement, and in the process captures the Khfiva. Durandal proceeds to destroy the rest the Battle Group and the S'pht'Kr arrive to defeat the remaining Pfhor on Lh'owon.
The Pfhor deploy the Trih Xeem, releasing the W'rkncacnter imprisoned within Lh'owon's Sun. The player was then sent through alternate timelines, looking for a way to stop the chaos. He eventually merges Durandal's core chip with Thoth, creating a hybrid A.I, referring to itself as we instead of I. It would speak 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".