From Halopedia, the Halo wiki
Anvarl, also known as the Planet of Blue and Red was a planet located in an unidentified binary star system. The planet earned its name because of its unique coloring, an effect created by the planet's suns. The San'Shyuum Reformists regarded the planet as one of the several wonders of the Milky Way galaxy.
Topography and ecology
Anvarl was located in a binary star system consisting of a blue dwarf star and a red giant sun. The two stars were perpetually positioned forty-five degrees above opposite horizons—the blue dwarf in the east, and the red giant in the west. Due to the interlocking gravitational fields of the two stars, Anvarl was motionless and did not rotate—resulting in the planet's lack of a day-to-night cycle. The constant pulling or repelling between the two stars would eventually tear the planet apart. The stars both provided the world with a unique hue; half the planet was "colored" blue and the other half was red. The "Purple Line" was a relatively narrow region on the planet where the two hues mixed.
Although it was substantial, the planet's gravity was eccentric. The planet's largest continent had rocky outcroppings and sand dunes shaped by gusts the planet's endlessly churning winds. The winds were constantly eroding the planet's rocks. The winds were partially caused by the planet's celestial alignment due to the opposed gravity that flexed back and forth. Near the Purple Line, there was several exposed Forerunner artifacts, with many more buried underneath other parts of the planet's surface.
The planet was once inhabited by the Forerunners. Numerous Forerunner artifacts were left behind on the planet. At some point, the planet was the homeworld of an unknown bipedal species that ultimately went extinct. The species had left behind carvings in ancient cities that suggested they had knowledge of the Forerunners.
At some point, the planet became inhabited by the Sangheili. During the War of Beginnings, Anvarl's location made it of strategic importance to the Sangheili. In 860 BCE, the San'Shyuum had discovered the planet and descended to the surface in an attempt to claim the world for themselves, after their Luminary confirmed the presence of Forerunner artifacts. The Skirmish of the Planet of Blue and Red was overseen by Hierarch J'nellin, while Mken 'Scre'ah'ben, High Lord of Sacred Relics, commanded ground forces on the planet's surface. The Sangheili forces were led by Ussa 'Xellus, who completely reorganized the planet's defenses upon his arrival. The Sangheili guarded the Forerunner artifacts from the San'Shyuum, although the San'Shyuum were outnumbered and not prepared for extensive ground combat. San'Shyuum scientists attempted to uncover relics, but were threatened by Sangheili assault teams. High Lord Mken ordered San'Shyuum-aligned Aggressor Sentinels to support the San'Shyuum's efforts. While Mken led the San'Shyuum from the relative safety of a force field, Sangheili forces were eventually able to disable the force field's generators by traveling through the various tunnels under the planet’s surface and detonating explosives to collapse the force field. Mken and other San'Shyuum fled to the Forerunner Dreadnought in orbit and prepared for orbital bombardment. Mken had ordered the Forerunner Dreadnought to fire upon the Sangheili forces, but under Ussa's command, most of them had escaped into a Forerunner structure, which the San'Shyuum were unwilling to damage. Ussa would ultimately escape from the planet.
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