The Library's true function is unknown at present. However it is hypothesized to be a collection of all information regarding the Flood known to the Forerunner and also collated cloned DNA sequences. All of this information is contained within the Index, which is housed within the Index Chamber and is used in the process of activating the Halo system and wiping the galaxy clean of sentient life.
The Index is suspended within the core of the Library, out of reach of possible Flood outbreak. The only internal access is via a giant lift that descends from the highest level of the Library to the Index's suspended position.
Access to the Library itself usually occurs via each installation's internal teleportation grid. On Installation 05 2401 Penitent Tangent was unable to assist the Arbiter in this manner. As a result a longer, less direct route had to be taken. An Anti-Gravity Gondola spanned the distance between a Forerunner base and the Library. This large Forerunner base was the Sentinel Wall which was protected by an energy shield and filled with hundreds if not thousands of Sentinels and Enforcers. Then, beyond the wall is an artificial lake and a great chasm that the gondola must cross.
On both Installation 04 and Installation 05 the Flood was still able to breach such defenses and infest the Library. This may indicate that the Library is not as secure as the Forerunners had hoped it would be, or that it was initially a containment facility for the Flood. The interior of both Libraries consists of multiple gigantic hallways of geometric design interconnected with one another, finally leading to the Index Room. The large hallways suggests that it was designed to hold large numbers of Forerunners, possibly acting as a backup refugee camp against the Flood.
Although only observed once, Libraries are not limited to Halo Installations, but are used on other Forerunner colonies as well. In the instance seen, Libraries on other worlds are surrounded by massive walls, and surrounding the building inside the walls are several Forerunner warships, ready for launch. It was in this Library that the Forerunners held a meeting and analysis on the new Flood threat.
It is here that 343 Guilty Spark remarks upon both the nature of the Flood and the Forerunners. Making specific reference to their technological superiority, he wonders why John had brought "such ineffective weapons to combat the Flood." Guilty Spark also recommends that John-117 should upgrade from his current Mjolnir Armor, a Mark V (which according to a Forerunner ranking system is only Class 2) to a Class 12 Combat Skin, which is stated as a necessity in order to combat the Flood. However, he later tells John-117 that his suit will serve him well as the Flood would try to alter the atmosphere for their own needs (possibly referring to the Flood spores spreading throughout the facility).
The Flood had heavily infested this library in the few short days since their initial breakout, with large groups using the ventilation shafts and access corridors to make their way through the facility. There were Flood in almost every part of the complex, including the Index Chamber itself, and the Sentinel drones assigned to the facility were mostly overwhelmed. By the time John arrived, the Sentinels were reduced to small groups using the shafts to avoid destruction and the Flood had begun terraforming, spreading their spores through the facility by using the ducts.
Toward the end of this Library, John encountered the body of a Marine who was Guilty Spark's previous Reclaimer. This Marine was said to have been so mutilated that the Flood could not possibly infest him. He was identified to be Staff Sergeant Marvin Mobuto.
It seems the Covenant made no attempt to access the Library on Installation 04. While John-117 explored the dark bowels of the structure, there is no sign of the usual crates, shield generators or spare weapons. This is probably because the Covenant did not find out about the Library, the Index or its purpose until after the capture of Monitor 343 Guilty Spark.
With help from the Sentinels, and motivation from Staff Sergeant Mobuto, John-117 fought his way through three levels of the complex and retrieved the index. The Library's subsequent status and Flood infection is unknown, although it was destroyed with the rest of Alpha Halo when John-117 detonated the Pillar of Autumn's engines. The library is not seen from the outside, so it is unclear whether the libraries on each ring are different. e.g. Installation 04's control room is different than Installation 05's.
Although it is never seen, it can be assumed that the Library of Installation 08 is almost identical to that of its predecessor with the exception of more elaborate interior designs. One can infer this for the simple reason that the Control Room is identical between the structures as well.
While John-117 hunted down the Prophet of Regret, Commander Miranda Keyes, Sergeant Johnson and most of In Amber Clad's Marines attempted to break into the Library in order to find and retrieve the Index. Due to the significantly larger Flood outbreak on this Installation, the Library and its defenses were seriously damaged and as a consequence were filled with a larger number of Flood than its Installation 04 counterpart. The UNSC forces were seemingly unprepared for the Flood presence, and many of them were infected by the flood.
The Arbiter traveled to the Library of Delta Halo. (This journey encompasses the Sacred Icon and Quarantine Zone levels.) Dropped into the Sentinel Wall from Tartarus' Phantom, the Arbiter battled his way through swathes of Flood and Sentinels to find the gondola that would carry him into the core of the Library. By removing the containment plugs, he then lowered the Containment Shield guarding the Quarantine Zone enabling him to enter the Library proper. Unfortunately, this also released the Flood and allowed them to gain access to victims and vehicles from both Covenant and human forces, adding to the magnitude of the outbreak.
The Arbiter took the gondola into the depths of the Library to find the Index Chamber. This inner section of the Library was identical to Installation 04's. After the Arbiter knocked Johnson unconscious, Tartarus and his Brute forces arrived. Tartarus pulled Keyes toward him with the Fist of Rukt, stunning her in the process, and removed the Index from her possession. Tartarus then betrayed the Arbiter, pushing him into the central shaft of the Library using the Fist of Rukt's gravity manipulation powers.
After being thrown to his death by Tartarus, the Arbiter is rescued by the Gravemind, who then held him captive for his own motives. It should be noted that in the Index Room the elevator is broken on the bottom level (opposed to being at the top like in Installation 04's), there is also what appears to be multiple destroyed Enforcers, as well as a number of the Gravemind's tentacles. A large part of the Gravemind exists beneath the Library itself, and this is where 2401 Penitent Tangent and the Flood Infected Prophet of Regret were held. It seems that the Flood just tossed dead bodies over the edge of the main shaft, possibly to gather biomass in order to form the Gravemind.
- The foremost structure of the Library is similar to the symbols on the stone slabs on the top floor of the bases in the Halo: Combat Evolved map Blood Gulch. It could mean that the structure of the Library is somehow related to Blood Gulch, or that it is purely a common Forerunner symbol.
- In the Halo 2 multiplayer level, Containment, if one flies a Banshee, in the distance Installation 05's library can be seen.
- In Halo: Combat Evolved, John-117 gets in the Alpha Halo Library through teleportation by 343 Guilty Spark. In Halo 2, the Arbiter goes through a gondola system to get in the Delta Halo Library, it may be possible that you can get into the Alpha Halo Library as well through a gondola system but it is unknown for sure.
Halo 2: Anniversary