Catalog

Catalog was a collective entity of Forerunner Juridical agents charged with recording, observing, and uploading information to the Juridical network. There were many individuals identified as Catalog, all of whom were formally considered to be the same being as a peculiar cultural convention.

Role and jurisdiction
"I do not judge. I record."

- Catalog

Catalog's task was to record evidence related to a given case, and to determine what would be passed on for the Juridicals to analyze. Catalog gathered information in a variety of ways, including direct observation via their various sensor equipment and taking the testimonials of individuals connected to the cases at hand. When on an official mission mandated by the Ecumene Council, no one, no matter their rank or position, was allowed to interfere with Catalog's investigation. Catalog was almost constantly connected to the Juridical network, which it used to share information. The Gravemind, upon capturing Catalog, noted an "amusing" similarity between Catalog and itself; much like the Flood's pervasive collective consciousness, Catalog was an extension of a collective which shared memories and experience.

Catalog was authorized to force a key deponent to divulge information through artificially-induced "calming encouragement" if they refused to willingly continue their testimony or otherwise cooperate. When three Catalog units gathered to record a particularly significant event, they would become a "triad", forming a local personal network that shared all information; "one unit with three points of view".

Physical appearance
Catalog was encased in a mechanical carapace designed for optimum observation and recording, and allowing it to interview quickly and efficiently while contacting the network as often as necessary. This carapace was also capable of surviving in hard vacuum and other dangerous environmental conditions. The Catalog carapace possessed five legs and an array of optical recording devices it could extend or retract at will. Like all Forerunners, Catalog was equipped with a personal ancilla.

When assuming the carapace, the biological body of Catalog was reduced into a shriveled, misshapen form. The carapace also acted as a life-support system for the withered body within; if Catalog chose to disconnect the carapace, it would suffocate, which was considered an honorable death in the event of failure. Catalog was conspicuously distinct from ordinary Forerunners due to its exotic physical shape; it also made random internal "clicking and slopping" noises which some Forerunners, including the Ur-Didact, found disgusting.

Personality and behavior
Catalog were chosen from volunteering individuals of all rates who had committed crimes and assumed the role as an act of penance. Forerunners who became Catalog were stripped of individuality and personhood; in translations, Catalog is referenced with the neutral pronoun "it". Some Forerunners occasionally referred to Catalog as individuals (e.g. a Catalog as opposed to Catalog), which was considered improper but forgivable. Committing to life as Catalog was known as "assuming the carapace". This process, which few outsiders truly understood, involved many oaths and heavy conditioning in preparation for the role.

While Catalog generally appeared to outsiders as being identical in mannerisms and personality, each individual's distinct origins inevitably resulted in subtle differences between their behaviors, sometimes enough for an attentive outsider to be able to discern the rate of origin of an individual Catalog. However, Catalog was not supposed to reveal its origins or its individual reasons for assuming the carapace to others, as its former identity was supposed to have been suppressed in favor of that of Catalog.

Due to their excessive involvement with legal matters, Catalog often appeared somewhat paranoid in regards to potential crimes, and had a tendency of phrasing and understanding everything in legal terms. Catalog used the phrase "I am Catalog" as a personal mantra, occasionally proclaiming it aloud to formally declare its presence.

History
"I am one of many. We are all the same. In theory."

- Catalog

During Master Builder Faber's control over the Old Council, many Catalog and other Juridicals were subverted to be part of the Builders' schemes, either willingly or covertly by the Builders, who supplied their carapaces. After the establishment of the New Council, corrupt Juridicals were exposed and punished. Near the end of the Forerunner-Flood war, Catalog collected testimony from a number of influential Forerunners; the Librarian, IsoDidact, Ur-Didact and Faber, in order to find evidence of possible crimes against the Mantle.

Catalog was assigned to Erde-Tyrene to collect testimony from the Librarian and the IsoDidact. This individual was formerly a Warrior-Servant, and became Catalog after going against the orders of their mentor out of anger. However, it took the carapace after the Master Builder's rule in the Council ended, and did not participate in the human-Forerunner wars. After the IsoDidact departed to command defensive operations of the Orion complex, this Catalog recorded the Librarian's testimony about her journey to Path Kethona.

Another Catalog had been assigned to Maker-of-Moons to collect testimony about the Master Builder some time earlier; this unit was later heavily damaged and abandoned by the Master Builder's forces on a Builder hulk near Uthera Midgeerrd. A former Miner, this individual assumed the carapace after prematurely triggering a planet's destruction, killing his crèche-mate and twelve of the latter's crew in the process; he did this out of jealousy toward his crèche-mate, who had been chosen to be married with the heir to the Miner rate's most powerful family. It chose to remain on the hulk with the Ur-Didact when the ship was surrounded by the Flood; as a result, the Didact considered it honorable, even appointing it into an honorary Warrior-Servant. It was subsequently captured by the Gravemind which latently infected it with the logic plague, intending to send it back to the heart of the ecumene to spread the software infection to other Catalog units and ancillas.

Catalog collected testimony from the Ur-Didact after his recovery from the Burn, being forced to compel the Didact to recall his encounter with the Gravemind after he refused to continue his testimony due to his reluctance to relive the horror he suffered at the clutches of the Flood. Catalog was also assigned to the Ur-Didact on Nomdagro in anticipation for his meeting with the IsoDidact and the Librarian; this unit formed a triad with two other Catalog assigned to Bornstellar and the Lifeshaper when they arrived. Catalog also attended the gathering of the Forerunner leadership on the greater Ark, and was likewise present in Omega Halo's control room when the ring was fired. This unit was saved from the collapsing control center, along with the IsoDidact, by Monitor Chakas aboard his Gargantua-class transport. It was slightly damaged but ultimately survived and made it to Installation 00.

Catalog assigned to the Librarian followed her aboard the Lifeshaper's personal vessel Audacity to Requiem and later to Erde-Tyrene. There, it recorded the Librarian's encounter with the Gravemind's messengers who revealed her the truth about the Domain. It also relayed her final message to the IsoDidact on the lesser Ark via the Juridical Network, perishing moments later when the Halo effect struck Earth.

Over a hundred thousand years after the activation of the Halo Array, the UNSC discovered and studied a Catalog unit designated as "Forerunner remains #879", recovering some of the records stored in its carapace and together with data from a Juridical monitor, piecing together an account of the final years of the Forerunner civilization.

A Catalog unit, having been dormant for 100,000 years after the firing of the Halo Array, resumed its duties on November 7, 2552, after receiving and denying a control authorization supposedly sent by the Librarian. After his awakening in July 2557, the Ur-Didact repeatedly contacted Catalog and asked to take control of the ecumene; Catalog rejected these requests. This prompted Catalog to resume its investigative protocols. Over the course of the investigation, Catalog accessed a number of UNSC and ONI data networks to gain information about the current state of the galaxy.

Trivia

 * Catalog's UNSC identifier of "Forerunner remains #879" suggests that 878 other Forerunner corpses, or parts thereof, have been discovered by the UNSC.
 * 343 Industries posts in-character in the guise of  on Halo Waypoint. From a metafictional standpoint, these posts represent an information exchange as part of Catalog's investigation after its reawakening in the modern era, the "present day" of the posts being March 2558; however, Catalog has noted that its current date does not advance at the same rate as time in the real world. In addition to answering users' questions about canon, Catalog's posts contain communications and maintenance logs which appear to originate from both the endgame of the Forerunner-Flood war and the modern era, including Catalog's attempts to establish communicate with a Forerunner facility on Mars as well as the Domain, and what appear to be communiques from Catalog's superior. The Catalog character  that its answers should be considered canonical at the time of posting, but that their specifics may be altered by newer media at a later time due to "reconciliation effects" and "integrity checks". See here for the wiki's archive of Catalog's posts.