From Halopedia, the Halo wiki
Forms were different physical stages assumed by the Forerunners over the course of a lifetime. The Forerunners were able to mutate into various forms through an artificially-induced transformation. Manipular, or Form Zero, was the first and simplest form, assumed by Forerunners at birth. It was noted that Manipulars bore a striking resemblance to humans; however, this would change with higher forms. The original anatomy of a Forerunner would also be distorted when mutating to higher forms, which typically had stiffer facial muscles than Manipulars and thus a more limited capability to convey emotions; for example, with certain higher forms, the ability to smile was lost almost completely.
In each mutation, a Forerunner would acquire the genetic imprint of their mentor, typically the individual's parent. However, Warrior-Servants sometimes performed "brevet mutations" in battlefield conditions, where the imprint would be supplied by one's commander. In rare cases, a brevet mutation could fail and result in deformities, although cosmetic changes or corrections could be made to ameliorate the damage to some extent. There were rumors that Forerunners whose mutations had failed were sequestered in special enclaves and hidden from the general population.
The form one would mutate into was dictated by their rate, the person who performed the mutation and the affiliation of the ship or other facility where the mutation was performed. For example, Warrior-Servants were noticeably taller, more rugged and muscular than Builders, whose forms were more refined. Aside outward appearances, there were also differences in mental and neurological functions. Higher forms of Warrior-Servants, for example, had extraordinary mental abilities; they could absorb massive amounts of information from one another in an instant, share sensory input with thousands of warriors during battle, or splinter their perception of reality into multiple simultaneous streams with alternating flows of time. While typically, the conditions of a mutation were carefully chosen to reflect the individual's rate of choice, abnormal circumstances could result in a peculiar mixture of features. Bornstellar Makes Eternal Lasting originated as a Builder and his imprint was supplied by the Didact, a Warrior-Servant, in a Lifeworker-provided vessel; as a result, he initially resembled a blend of all three rates, before the Didact's Warrior pattern settled in on him.
A new mutation pattern was being introduced in the final decades of the Forerunner civilization. Instead of being subjected to multiple mutations over several centuries, one would undergo a series of changes over a single year. This would result in fewer inherent physical or mental distortions. Most Forerunners mutated according to this new pattern came from Builder families, with a small number of Warrior-Servants among them.
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