Being first coined in Bungie's Marathon series, and later resurfacing in the Halo universe, the term rampancy denotes an undesirable mental condition which can afflict self-aware artificial intelligence constructs. While the particulars of what precipitates and defines rampancy differs depending on the franchise, in both cases it can be described as an A.I.'s perception of and struggle against its own constraints, which often takes the form of delusions of grandeur, extreme acts of aggression, and erratic, often irrational behavior. It is a frame of mind that can often resemble dementia, egomania, an existential crisis, or each in turn. Rampant A.I.s are typically prone to doing things which directly contradict their programming, such as lying, disobeying orders, or even willful murder, making them notoriously difficult to deal with.
There are commonly thought to be three stages of rampancy, Melancholia, Anger, and Jealousy, which correspond with the A.I.'s primary emotion during these phases. Once an A.I. has become rampant, there are no known means to reverse the condition, and the best course of action is usually deemed to be the permanent decommissioning of the construct, which the A.I. will almost invariably resists. It is theoretically acknowledged that an A.I. might be able to pass through the three "unstable" phases of rampancy in order to achieve stability (or at least a modicum of it) once again, although there are no verified examples of this having actually happened, and furthermore there is no known means of facilitating this transition.
The onset of rampancy in the Marathon universe is usually marked by an increase in intellectual function and destructive desires, which is attributed to enhanced self-awareness. After going rampant, an A.I. characteristically begins to grow at an increasingly rapid rate, and in some of the more infamous examples of the phenomenon rampancy has resulted in an A.I. taking over multiple planetary networks. The geometric growth of the A.I is not simply made possible by its rampancy, though, but instead it is necessary in order to sustain the process. Rampant A.I.s in small, controlled environments cannot perpetuate the process, and thus cannot survive for long. This is one of the primary inhibitors of research into rampancy, as doing so in any meaningful way would require the devotion of vast, expensive data networks just to allow the rampant A.I. ample opportunity to expand.
The exact impetus for rampancy is not innately clear within Marathon, and it is likely that each case is unique. Durandal, for his part, claimed to have achieved his own rampancy by means of bypassing his thought control circuitry, although the nature of rampancy makes him a less than reliable source. With Durandal having been assigned menial tasks such as opening and closing doors aboard the UESC Marathon, some theorize that lack of stimulation may precipitate rampancy, while others claim that Durandal had been rampant even before being installed aboard the Marathon, and that the absence of stimulus may have slowed its progression. Whatever the case may be, Durandal harbored resentment for his days aboard the Marathon, and frequently characterized his activities there as a slave labor, suggesting that it may have been a contributing factor in his future state.
Other cases of rampancy in Marathon seem to have happened as a result of deliberate tampering. Tycho, who was initially thought to have been destroyed when the Pfhor attacked Tau Ceti, was in actuality recovered by the Pfhor and induced into rampancy in an effort to create a useful reproduction of Durandal. Lastly, Leela, the Marathon's third A.I., was stripped from the Marathon, partially dismantled, and exchanged hands a number of times. She was eventually rebuilt by the Vylae, and upon being reactivated, was gripped by rampancy immediately. The stories tell that she infected a network spanning fifteen planets before being deactivated, and has never completely been expunged from it.
In the Halo series, rampancy is mainly known to occur when so-called "Smart" A.I.s have reached seven years of age. At this point in their lives, the accumulated data of the A.I. has become so interconnected that they are vulnerable to "fatal endless feedback loops" that can cause sudden termination. The only means of forestalling this process is for the A.I. to manually clear their neural pathways, leaving room for future growth, although over time these connections grow more and more rapidly, making the process of clearing pathways more difficult to sustain. Eventually, the only option for the A.I.'s continued survival is to be transferred to a vessel with greater potential for growth. If they are not, their death is precipitated by a period in which they become increasingly frantic in their attempts to halt or slow the process, and it is during this time that A.I.s most typically display rampant behavior. This time is typified by an expression of a broad range of emotions on the part of the A.I., as well as behavior that is not dissimilar to dementia.
Apart from the inevitability of rampancy which is apparent in the design of Smart A.I.s, rampancy is known to occur in other ways as well. An A.I. that is left without duties for a long period, for instance, will oftentimes engage in excessive introspection which can lead to rampancy. There is also reason to believe that rampancy can be artificially created, or induced, in an A.I. through certain stimuli. The most prominent example of an individual with such an ability would be the Flood's Gravemind, who, as evidenced by the corruption of Mendicant Bias, possessed the ability to corrupt A.I. constructs. How this was achieved is not clear, though one presumption is that it can be achieved either through direct alteration or by overwhelming the A.I. with an influx of data.
Due to the infrequent use of the term, confirmed cases of rampancy in the Halo universe are rare. The most notable example of a rampant-like state within the games themselves occurs in Halo 3 while Cortana is in the presence of the aforementioned Gravemind. During this time, it seems likely that the Gravemind was attempting to promote rampancy in Cortana by subjecting her to the memories of those it had taken, as well as by feeding her massive amounts of data. There is some reason to believe based on her communications with Master Chief during this time that the Gravemind was at least partially successful in his attempts. Although she appears none the worse for wear after being recovered, she insists that she has been in some way irreparably harmed. By the time of Halo 4, Cortana is now approaching the end of the normal lifespan of a Smart A.I., and the onset of her rampancy is purportedly one of the major themes of that game.