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Were you looking for Huragok, the species often known as Engineers, or the Engineer MJOLNIR variant?
Prior to the release of the Halo: Fractures story Promises to Keep, the Engineer rate was only referenced once in Halo: Cryptum, when Bornstellar states that all rates "except for the Engineers" were represented in the Capital Court; he regards the Engineers' exclusion from the proceedings as an obvious fact and does not elaborate on the reasons thereof. According to Greg Bear, the author of Cryptum, this usage was meant to refer to the Huragok, who are mentioned earlier in the novel as being allowed in Forerunner company only rarely, as opposed to a hitherto unmentioned rate of Forerunners. Promises to Keep establishes the Engineers as an actual rate of Forerunners, contradictory to Bear's original intent. The Engineers' dissociation from the rest of the rates mentioned in Cryptum is not elaborated upon, and there does not appear to be a discernible social gap between the Engineer featured in the story, Walking-in-Light-of-Falling-Stars, and members of the other rates.
List of appearances