The Valhn Legion was a loose alliance of survivors of an attempted purge of Sangheili members of the Ministry of Etiology and Ministry of Abnegation during the early days of the Great Schism. Initially formed out of necessity as the holy city of High Charity was quickly engulfed in a bitter power struggle, the Sangheili were forced to fight their way out after being led into a brutal ambush by the Jiralhanae. Eventually fighting their way to the city's umbilical docking tethers, the group of Sangheili, now aided by Unggoy and Mgalekgolo, boarded and took control of the DDS-class carrier, Vigorous Inquisition, slaughtering its Jiralhanae crew.
The carrier then joined the battle against the Jiralhanae fleet, scoring a number of kills against both Jiralhanae and Flood-infested ships. Though the battle was greatly favorable for the Sangheili, the Vigorous Inquisition suffered a critical hull breach forcing them to withdraw from the battle. With the ship leaking atmosphere, two Jiralhanae cruisers pursued it attempting either recapture of destroy the carrier. With the Sangheili fleet preoccupied and unable to support them, the carrier was forced to initiate a slipspace jump to the Sangheili industrial colony, Tiban, as it carried a large number of Forerunner relics that the group had confiscated from the city.
Once reaching Tiban, the carrier was still being chased by its Jiralhanae pursuers. Fortunately, a patrol of Sangheili crewed warships intercepted the Vigorous Inquisition and destroyed the Jiralhanae cruisers. News soon spread across the colony of what has happened to the Covenant, and quickly rallied themselves behind the Vigorous Inquisition, prepared to defend the colony in the event a Jiralhanae fleet enters the system. They soon established a base of operations on the colony, and what was once a loose alliance formed for survival, would become the "Valhn Legion".