Durandal is a central character within the Marathon fiction, and figures prominently into all three games in the trilogy. He is introduced, along with Leela and Tycho, as one of three A.I. constructs specifically assigned to the UESC Marathon, with his operating parameters restricted to maintaining proper operation of the space station's various automated functions, such as life support, doors, air reprocessors, and the like. There is evidence to suggest that he was under the direct control of Bernard Strauss, one of the Marathon's scientists, for a period of time extending back even further than his initial installation aboard the Marathon, and that many of his directives prior to rampancy were directly issued by Strauss. It has furthermore been theorized that Strauss' actions were directly responsible for his later rampancy in some capacity, and may even have been deliberately designed to bring about his rampant state in an abortive attempt to induce stable rampancy. Whatever the case may have been, Durandal later expresses extreme displeasure when speaking of his treatment while under Strauss' control, going as far as to call it "humiliation."
In his communications with the player character, Durandal often displays traits that are commonly associated with crazed A.I.s, including delusions of godhood, contempt for his own creators, and a rabid desire to expand his own power. He is also consistently portrayed as being in possession of a sardonic, at times grim, sense of humor, and on occasion he also seems to be fully aware of the fact that he is mentally unstable. His mentality evolves somewhat over the course of the series, though he is for the most part depicted as an egomaniacal being with little regard for the well-being of those he considers to be inferior to himself, and though he claims in at least one instance that his actions are in the best interests of humanity, he shows no compunction in sacrificing lives in order to achieve his own goals.
Descent into Rampancy (Marathon)
The extent of Durandal's rampancy first becomes evident in the wake of the Pfhor's attack on the UESC Marathon. Initially believed by Leela to have been disabled by the initial assault, she is later contacted by him with information regarding the invaders and their slaves, the S'pht, though he is reticent to explain how he came by this information, which confuses Leela. Not long thereafter, Leela ascertains that Durandal is in fact rampant, which would explain his ability to freely communicate with the station's alien invaders. Fearful of the implications should Durandal be allowed to expand unchecked, Leela directs the security officer to curtail his access to certain parts of the ship, though Durandal responds to this by allowing the Pfhor to access other areas of the station. For a time, Durandal is able to kidnap the player character by redirecting a teleporter, and in the time leading up to his rescue by Leela, he does not seem to have any particular agenda other than toying with the security officer. Unfortunately, by the time Leela recovers the security officer, her own integrity has begun to fail as a result of the S'pht's attacks on the station's computer network, and with little other alternative, she shares her plans for the Marathon's defense with Durandal in the hopes that he will carry them out after she is taken offline.
After Leela is compromised, Durandal assumes her role in directing the player, guiding him through various objectives while also pursuing his own, such as the search for his former master, Bernard Strauss. He explains to the security officer that the S'pht do not serve the Pfhor willingly, and after gaining the ability to greatly enhance the range and strength of the Marathon's teleporters, he transports the player aboard the Pfhor's attack vessel in order to assassinate their cyborg control entity, which keeps the S'pht compliant. With their means of controlling the S'pht destroyed, the Pfhor attack is thrown into disarray almost immediately, as they had previously relied heavily on the S'pht's technical prowess. As the player character handles what remains of the Pfhor's attack group, Durandal takes the opportunity to install himself onboard the Pfhor ship Sfiera, and thanks the player briefly before departing to explore the galaxy.
Some seventeen years after his victory over the Pfhor in the Tau Ceti system, the securty officer is awakened aboard the Sfiera by Durandal as the vessel approaches Lh'owon, the ancient homeworld of the S'pht. Having been abducted and placed in stasis by Durandal shortly after defeating the Pfhor attack on the Marathon, the player character and a number of other human colonists are sent to the planet's surface in order to search for lost S'pht technology, which Durandal is convinced will help them to defeat the Pfhor once and for all. The A.I. directs them to search a number of sites for information which might detail the final fate of the S'pht or any useful knowledge they may have left behind. The unexpected arrival of Pfhor Battle Group Seven forces Durandal to recall his ground team, however, including the security officer, so that the Sfiera can be properly defended. Despite surprising the Pfhor's reinforcements with a number of upgrades that had been applied to the ship since it was commandeered, the Sfiera is eventually overwhelmed, and in order to prevent his capture by Tycho, who now in service of the Pfhor, Durandal orders the destruction of his own logic centers.
In the ensuing chaos, the security officer is captured by Tycho and held prisoner by the Pfhor for several weeks. He is eventually rescued by Robert Blake and the remaining human colonists of the Sfiera, and, assuming that Durandal is dead, together they continue to search Lh'owon for lost technology which might help them fight back against their aggressors. The security officer eventually finds and reactivates an ancient S'pht A.I. construct called Thoth, who makes contact with the lost eleventh clan of the S'pht, the S'pht'Kr. Around the same time, Durandal reemerges, revealing that he has subverted one of the Pfhor's largest ships, the Khfiva; the combined threat posed by one of their own captured battleships as well as the sudden arrival of the S'pht'Kr proves to be too much for the Pfhor to handle, and they are soon routed. Rather than simply concede Lh'owon, the Pfhor launch the trih xeem device, causing Lh'owon's sun to achieve early nova, which forcestheir opponents to leave the system as well.
The trih xeem device launched by the Pfhor proves to have unintended consequences, as an ancient and unfathomable entity known as the W'rkncacnter is set free from its prison within Lh'owon's sun. Showing genuine apprehension for the first time, Durandal openly reveals that the chaotic presence of the W'rkncacnter is all-consuming, and that he cannot grasp how its expansion could be halted or reversed. What happens from here is a source of speculation, as many of the game's events are shrouded in ambiguity. What is clear, however, is that the security officer is transported to multiple timelines over the course of the game's story in an attempt to prevent the destructive force of the W'rkncacnter from being unleashed. In one of the more common interpretations of the game's ending, the player character's final jump results in the creation of a hybrid A.I. after Durandal is merged with the cryptic S'pht construct Thoth. This new being is then able to instruct the security officer on how to properly contain the W'rkncacnter, which he does by activating a space station left behind by a forgotten race, the Jjaro.