Sarcophagus

The Sharpened Shield, known to the United Nations Space Command as ONI Research Facility Trevelyan, is a Forerunner Dyson sphere formerly located within the artificial planet Onyx. At approximately two astronomical units (the diameter of Earth's orbit) in diameter and weighing approximately 1.37 solar masses, the sphere is the largest known megastructure created by the Forerunners, and possibly the largest artificial structure in the galaxy.

Description
The Sharpened Shield is a Dyson shell, a massive artificial structure encircling a star. According to Dr. Halsey's calculations, the internal surface area of the sphere is approximately 500 million times the surface area of Earth, or around 255 quadrillion km2. When residing in a slipspace bubble of compressed dimensionality, the shield world's apparent diameter to an outside observer was only about 23 centimeters. In normal space, however, the sphere is approximately twice the size of Earth's orbit. The exterior surface of the sphere is entirely featureless, and has been described as being chocolate-brown in color.

The observed portion of the interior surface of the Sharpened Shield is verdant forest, with several large cities in between. As the Forerunners never arrived, the shield world is entirely maintained by Huragok and Sentinels. The Dyson sphere's default environmental conditions are suitable for Earth-based life, but the climate settings could be adjusted for other species' requirements.

There are two known sections of the shield world. The Citadel is an urban habitat that served as the base camp for Blue Team for the majority of their time in the shield world. Another section, known as the Sarcophagus, is essentially a garage filled with numerous Forerunner transports and vehicles. The Sarcophagus also contained a control room. It was in this section that Lucy-B091 discovered several Huragok maintenance personnel, and successfully communicated with one of them.

Access to the shield world while it is contained in slipspace can be obtained through a small portal, which was located inside an antechamber in the core of the artificial planet Onyx. The portal was only active for a limited amount of time after the Halo Array had been put into standby mode. After the portal has been deactivated, the Huragok crew can transition the shield world back into normal space if there are no longer any threats to the shelter.

History
The Sharpened Shield was constructed by the Forerunners during the Forerunner-Flood war as a fallout shelter from the firing of the Halo Array. Near the end of the Battle of Onyx on November 3, 2552, Covenant Fleet Master Voro Nar 'Mantakree led his forces in an attempt to access the Sharpened Shield. However, SPARTAN Kurt-051 detonated a pair of FENRIS nuclear warheads within the core room antechamber after his comrades had passed through the portal, denying the Covenant access. The human survivors, including SPARTAN-II Blue Team, several SPARTAN-IIIs, Dr. Catherine Halsey, and Senior Chief Petty Officer Franklin Mendez, were then seemingly trapped within the shield world. Shortly thereafter, the installation's Sentinels destroyed the artificial surface of Onyx.

When inside the shield world, the human survivors managed to establish contact between the ONI ships outside the sphere, which had already detected the cyclically fluctuating anomaly caused by the slipspace bubble but had no means to access it. In February 2553, the shield world was eventually brought back into normal space by a group of Huragok that the Forerunners had left behind to maintain the structure. There was a considerable difference in the passage of time between the slipspace bubble and real space; while the survivors inside the sphere had only experienced several days, approximately three months had passed on the outside.

Once the Sharpened Shield had been brought back into normal space, the ONI team Kilo-Five entered the sphere. A strip of multi-colored lights activated to guide their dropship to the designated coordinates, where they would land on the surface of the shell. After a momentary blackness and a disorienting sensation for the occupants, they found themselves and their ship inside a hangar facility within the sphere's interior. Halsey and the surviving Spartans were then rescued from the Sharpened Shield, but she was immediately arrested by Captain Serin Osman for "committing acts likely to aid the enemy". Meanwhile, ONI science teams were sent in to recover Forerunner technologies from the shield world, which was named "Trevelyan" in honor of Kurt-051, whose sacrifice allowed humanity to access it.

Several days later, the former Shipmaster Jul ‘Mdama, who was captured by Kilo-Five, was brought to Trevelyan for interrogation by Dr. Magnusson.

Trivia
The Sharpened Shield was originally a fan-made title used in Halopedia, without any evidence of the title being used in any Halo media. The official parties to the Halo franchise seemingly acknowledged this title as canon in their publication of the Halo Encyclopedia. For more, see this discussion.

List of appearances

 * Halo: Ghosts of Onyx
 * Kilo-Five Trilogy
 * Halo: Glasslands
 * Halo: The Thursday War