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The Xilo Tetrahedron is a region of space defined for its triangular pyramid shape. The tetrahedron has four vertex corners, and at each corner lies a solar system with bright stars. The three solar systems that make up the base of the tetrahedron have at least a single planet that has been colonized by the Covenant. Vesperum and Sdrala are two known planets located in their own corners of the Xilo Tetrahedron. The other remains unknown. The solar system that marks the tip of the tetrahedron, should a directional tip exist in space, has the brightest star in Covenant-patrolled space known as Xilo. Xilo doesn't have terrestrial planets orbiting it, just gas giants with some of the most beautiful rings in Covenant space.
The Xilo Tetrahedron contains several Forerunner Line Installations and is believed to have been apart of the Maginot Line. During the Forerunner-Flood War, these installations were used to protect Shehaqim or Shield 5040. Shehaqim contains an entrance portal where sentient beings can enter the Domain, and it was feared that the Flood would access the portal and corrupt the Domain. 02-1147 Lambent Glow is the Montior in charge of operating the Line Installations and Shield 5040. When the Flood breached the Maginot Line nearing the end of the war, Glow was forced to activate the Shield World and send it into Slipspace. The Line Installations were disabled after the Halo Array's activation. Glow went into extended dormancy until the Forerunners could return and activate him for duty.
The Covenant would come to colonize this region of space. Sdrala possessed several Forerunner structures, once Line Installations, that have now become the center of art and culture for the Covenant. Unbeknownst to them, 02-1147 Lambent Glow remains dormant on the planet. In 2553, the Jiralhanae Chieftain Fabdius would restore the dormant Monitor and come to an agreement with the Forerunner AI. Glow would assist the Covenant Remnant in their war against the Sangheili and in return, Fabdius would restore Glow's connection to the other Line Installations. The Prophet of Exuberance would name Fabdius as the Chosen One because he revived an Oracle.
Throughout the conflict, Glow would offer his Sentinels to aid the Covenant Remnant in naval and ground engagements. However, the Covenant Remnant would fall apart in August 2553, and Fabdius could not keep his promise to restore the Monitor to full capacity. In September 2553, Lambent Glow managed to interface with the Command Base on Vesperum. He sent out a signal that was meant to awaken the Forerunner Wyverns, but it would heard by the Relic Hunters and the UNSC Carpe Diem. Both groups raced to Vesperum to retrieve the Forerunner AI, but were shot down in orbit and crashed to the surface. The two groups descended into the structures in pursuit of the Monitor.
Glow activated the Armigers, humanoid robotic constructs, to defend the Command Base and himself. The Relic Hunters would reach the Monitor, but would be ensnared in a containment field. The humans would locate an interface where their AI, Meredith, could infiltrate the systems. The human AI was unable to overpower the Monitor, but it could gain access to the local cartographer. The UNSC headed to the Engineering Room and Meredith convinced the Huragok to disconnect the Montior from the Command Base. The Huragok did so in the belief that the fighting would subside, and this allowed Meredith to gain access to the entire system, though not access to the Wyverns.
However, the Relic Hunters were released from the containment field and disabled the Monitor. Meredith tried to use the Armigers to stop the Relic Hunters, but she was cutoff from the same Huragok who wanted peace. The Relic Hunters boarded a Cherub transport vessel that survived the crash and made their escape. Three hours later, the Sangheili aligned with Zuka 'Zulee arrived in the system. Meredith was left with one option: self-destruct the entire base to prevent Covenant access to Forerunner weapons. The humans escaped aboard a prowler that survived the crash, and the entire Command Base was destroyed.
Locations in the Tetrahedron
- Xilo: this is a bright star located at one of the four vertex corners of the Tetrahedron; it could be said that this is the tip of the Tetrahedron if one could exist in space. The Xilo System is said to have gas giants with the most beautiful rings. Unbeknownst to the Covenant, the Forerunners used these gas giants to hide their Wyverns, large interstellar combat machines.
- Vesperum: this planet is located at one of the four vertex corners of the Xilo Tetrahedron. The Forerunners used this planet as a Command Base for the Line Installations scattered about the Tetrahedron.
- Sdrala: this planet is located at one of the four vertex corners of the Xilo Tetrahedron. The Forerunners placed numerous Line Installations throughout the planet's surface, but these installations were deactivated and preserved after the Flood had broken through them long ago. Sdrala has become a monument to Forerunner culture and art by the Covenant.
- Okhotsk: this is a moon colony located outside the Tetrahedron. The Covenant colonized this world and used it to mine deuterium and tritium which are used in the plasma reactors.
- Shehaqim (Shield 5040): the Xilo Tetrahedron is its own Maginot Line with one purpose, to protect Shield World 5040. This artificial world possesses a rare portal that can grant someone access to the Domain. The Line Installations were created to prevent dangerous or unwanted individuals from accessing the Domain in this form. The Flood were one such threat, and the Gravemind was able to break through the defense and almost reach the portal. Before that, 02-1147 Lambent Glow activated the Shield Installation and sent it into Slipspace, somewhere safe from the Flood. Since then, the Line Installations and the Monitor have been deactivated and remain dormant.