From Halopedia, the Halo wiki
Ancilla was a Forerunner term for an AI construct. They came in several varieties, each designed for different purposes and possessing differing capabilities. Although the term was often used to refer to all Forerunner AI, ancillas and monitors were sometimes referenced as distinct categories, monitors being defined by their floating robotic shells whereas ancillas, in general, were disembodied.
Ancillas were ubiquitous in Forerunner society, and were housed within personal shells like those of the monitors, as well as personal armor, ships, buildings, and various forms of machinery. Ancillas possessed varying personalities, often approximating that of a Forerunner. They were categorized into various types, including a system of classes; third-class ancillas were known to be issued to the plasma jockeys, a class of Forerunner stellar engineers. Monitors were a common type of robotic ancilla employed in many roles. The most powerful ancillas, next to the Contender-class, were the metarch and sub-metarch varieties. Some ancillas designed to interface with Forerunners typically represented themselves via avatars similar to those used by human AIs. Not all ancillas assumed visible form, such as those that operated within a metarchy; a vast network of ancillas coordinated by a metarch.
Ancillas were also stored within a Forerunner's personal armor and could be accessed by the user at will for information via a form of neural link. Most of these personal ancillas assumed the form of a blue female Forerunner, manifesting itself in the "back of the wearer's thoughts". An ancilla's appearance would be modeled according to the species of the individual wearing the armor; for example, when Chakas and Riser received suits of Forerunner armor, their ancillas appeared as a female hamanune and chamanune, respectively. Personal ancillas were typically not named and were addressed only as "ancilla".
For a Manipular, a personal ancilla made itself easily visible and readily accessible to provide guidance and education. Once Forerunners mutated to first-form, accessing the armor's ancilla appeared to require more effort, as it was expected that a mature Forerunner did not require as much attention as a Manipular; this also allowed the ancilla to divert more of its processing power to other tasks. Ancillas also helped Forerunners access the Domain after mutation.
The Forerunners viewed ancillas as little more than tools and programmed them to not only obediently perform their designated tasks, but, at least in some cases, also enjoy their servitude. The notion of an ancilla rebelling against its masters was virtually unprecedented, although this changed with the emergence of the Flood's logic plague which could effectively infect and convert Forerunner AIs. Like their organic Forerunner masters, ancillas would recite a mantra or prayer before imminent termination.
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