Requiem

Requiem is a Forerunner shield world located in the Milky Way galaxy. It once served as the central base of operations for the Prometheans, an elite class of Warrior-Servants, for thousands of years until the end of the Forerunner-Flood war. It remained sealed off for a hundred millennia since the end of the war.

Forerunner era
Requiem once belonged to the Warrior-Servants, the Forerunners' designated military class. It once served as the home of the Warrior-Servants' Promethean class, and was a central hub for all Forerunner forces and war efforts. It was sealed during the Forerunner-Flood war by the Librarian, who imprisoned the Didact in the core of the shield world to prevent him from using the Composer on all of humanity and turn them into war machines in his army.

Rediscovery
In 2554, Jul 'Mdama and a Covenant remnant faction discovered Requiem, using coordinates provided by Forerunner symbols. Over the next three years, the Covenant tried to enter the planet, without success.

In summer 2557, the severed aft section of the UNSC frigate Forward Unto Dawn, containing John-117 and Cortana, drifted near Requiem and were forcibly pulled inside an automated entrance portal on its surface, along with a number of ships from a Covenant fleet and later, the prototype starship. Inside, they encountered hostile Promethean forces that occupied the structure, and unwittingly released the Didact. The humans battled their way through the world, destroying its gravity well generator so the Infinity could escape. The Infinity escaped from the shield world, and despite the best efforts of the Master Chief and Cortana, the Didact escaped as well to go after the device known as the Composer, chased by the two in a Lich. Six months later, the Infinity returned to Requiem, facing both the local Prometheans and Covenant remnant forces which still occupied the structure.

Background
The most resilient shield installation built by the Forerunners, Requiem consists of multiple concentric planetary shells enclosed within one another, sealed inside an enormous outer shell.

Requiem has an external surface composed of layers of overlapping machinery, which can be seen externally by the number of lights on its surface. Some of these formations take on geometric patterns, such as circular orientations or arrays of lines. Amongst its complement of luminous formations are two massive symbols; the circular symbol for Reclaimers, and the symbol for the Mantle, which has also been seen on numerous Forerunner installations, such as the Halo Array and the lesser Ark, Installation 00.

A readily seen feature of this construct is that it harbors a number of large circular entrance portals, with iris-type opening mechanisms. When opened, a vibrant, blue-white light can be seen emanating from the interior as an artificial gravity well draws everything outside in if the presence of Reclaimer is detected. Though the full extent of the technology of the mechanical exterior shell has yet to be documented, it is known that as a type of Shield World, it is capable of protecting its interior from the effects of the activation of the Halo Array.

The interior of the facility is unique relative to the other known Shield Worlds; rather than housing an artificial star at its center, surrounded by a habitable inner surface, the exterior shell houses additional layered shells, the outermost of which has an environment resembling a terrestrial planet. Another important feature is that immediately above the cloud layer of the protected second layer, the interior surface of the protective shell is visible. This is demonstrated by the sky being dominated by the machinery above it, with large, vertical structures reaching down to the surface below.

Another distinguishing feature from other Forerunner artificial worlds is that Requiem's artificial star is located in the machinery above the interior planet. It is unknown how day and night cycles are implemented, specifically relative to the significant number of structures anchored to the planet's surface. The next known aspect of this facility is that the protected planet itself possesses fully active and functioning technology, unlike other Forerunner sites, which have largely been abandoned and dormant, with the exception of Shield World 006.

For travel between the layers and various locations, Requiem uses teleportation portals that transport anyone who uses them to different locations on the shield world. If the person possesses an AI like Cortana, that AI can control the portals. The Prometheans that occupy Requiem are capable of generating portals and translocating across the shield world.

Trivia

 * A requiem is a church mass celebrated for the repose of the soul(s) of one or more deceased persons. This may be a reference to the Forerunners' mass extermination following the firing of the Halo Array, or to the fact that John-117 has been presumed dead by the UNSC.
 * Prior to Halo 4, Requiem was known among fans as the Legendary Planet. This is a reference to the planet's first appearance in an exclusive ending in Halo 3, which is viewable only by completing the last level of the game on Legendary difficulty. In the ending, the aft section of the Forward Unto Dawn is seen drifting toward the planet; seconds later, a light shines across the surface of the planet, as the Forward Unto Dawn is caught by the planet's gravity. Requiem also appears in a small image in the Halo Encyclopedia, where it is shown to have a reddish-orange color on a small illuminated section. The planet appears again in the Halo 4 announcement trailer, where the aft half of the Forward Unto Dawn is seen being pulled into a massive, iris-like opening on its surface. The planet was finally identified as "Requiem" in the May 2012 issue of the GameInformer magazine.
 * An image of the planet is present in the bulletin board of the in Halo: Combat Evolved Anniversary, in a poster for a fictional film titled Dark Planetoid Rising.
 * On the surface of the planet, as seen in the Legendary ending for Halo 3, a third symbol can be seen alongside the Reclaimer and unspecified Forerunner symbol; The 7-spoke insignia of the Bungie fan community known as Seventh Column. The symbol for Reclaimer is also the insignia for Marathon, a game trilogy made in the years before Halo: Combat Evolved was developed.
 * According to Brian Reed, the lead writer of Halo 4, Requiem is the largest Forerunner construct in history. Unless he was speaking hyperbolically, this would mean that Requiem is even larger than Shield World 006, which is two astronomical units in diameter. If he meant that Requiem is merely the largest construct featured in the games, it would be larger than the 100,000 kilometer lesser Ark.

List of appearances

 * Halo 3
 * The Commissioning
 * Halo: The Thursday War
 * ''Scanned
 * Halo 4