From Halopedia, the Halo wiki
Luminal Beacons were small Forerunner devices, capable of tracking their own location through gravity-wave analysis and temporal distortion. This was done thanks to quantum-dot processing machines and integrated senors within the beacons. Using this, and quantum enganglement, the devices were able to instantly relay their locations to reception units within High Charity. They were considered so advanced that even the Ministry of Discovery was unable to reverse-engineer or understand them.
Created by the Forerunners, these devices would eventually be found by the Covenant during their thousands of years of searching for Forerunner relics. By the year 2525, only four of these beacons had been found by the alien hegemony. Due to this and their abilities, they were considered deeply sacred, kept within the Beacon Keeper's Vault, along with their reception units. Their sole purpose within the Covenant was to be given to fleets heading to unknown space to announce the presence of a Halo ring, if one was found. More then five thousand cycles prior to 2525, a Covenant fleet, with one of these beacons vanished, ultimately transmitting from deep within the galactic core. Thanks to the dense star density and strong gravity, it was beyond the Covenant's ability to retrieve and was called the "Fabled Lost Beacon" afterwards
During the first months of the Human-Covenant War, the Hierarchs entrusted two of the three remaining beacons with fleetmaster Nizat 'Kvarosee on his campaign into human territory, as they claimed clues to finding the Halo rings could be found there. During the Attack on Zhoist, one of these beacons was lost when the ship it was on, Almighty Persuasion was destroyed. The remaining one would be taken back to High Charity when Nizat was brought back for debriefing. Here he would suggest using it as a way to wipe out ONI
When it was found out one had been lost under his charge already, the plan was declined and he was ordered to return the remaining beacon to the Beacon Keeper's Vault. Here, Nizat, his second, Tam 'Lakosee, and nine other shipmasters would kill the guards and the San'Shyuum Beacon Keeper himself in order to obtain the other remaining beacon, along with the reception units and use them both to find and destroy ONI. During the Battle of Netherop, one of the beacons, hidden in a human emergency locator, would be planted on a ONI agent, while the other, unbeknownst to the humans was stored in armor with a personal energy shield, and subsequently captured by them as well. Meanwhile, their reception units were aboard the stealth corvette Quiet Faith as it managed to flee the system.
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