From Halopedia, the Halo wiki
At some point during or before January 2552, the archaeological colony of Ariel came under attack by the Covenant. Led by a Lesser Prophet, they sought a Forerunner construct referred to as the Knowing. One of the colony's civilians managed to send a distress call, to which the UNSC responded with sending ODST forces to investigate on the 8th.
After deploying to the planet, the ODSTs found a body in the transmissions bay: a man had apparently died of fright. Other civilians or bodies were nowhere to be found. The ODST squad under the command of MSgt Frost split into two-man teams, with each team investigating a different area of the colony. Frost and Michaels investigated the areas closer to the surface, with Frost contacting the other teams regularly. Dutch and Romeo proceeded into the generator complex of the colony, where they observed several Elites and Grunts digging for something in the generator room. They reported their findings to Frost, who ordered them to get back topside.
Higgins and Campbell investigated the motor pool, where they were caught by surprise and killed by several Jackals. Meanwhile, Dutch and Romeo were suddenly engaged by massive numbers of Grunts and some Elites, who cut off the troopers' exit route. Romeo managed to collapse a platform, killing two Elites. However, they were soon engaged by another Elite, who overpowered them in close combat. Meanwhile, one of the generators went critical, starting a chain reaction that would destroy the whole complex. Frost ordered everyone to evacuate, leaving Dutch and Romeo behind. On foot, they then proceeded to the colony's other archaeological dig, situated some kilometers away.
Unbeknownst to the other Marines, Dutch and Romeo survived the explosion, and sought to regroup with the rest of the ODSTs. After finding their way to a higher ground, they noticed a large Covenant ground force amassing behind the other ODSTs, still on their way to their destination. The two decided to lure the Covenant away from their compatriots, detonating several explosives jammed into a fissure and burying numerous Covenant under a collapsing canyon. Dutch and Romeo then escaped, planting traps for their pursuers as they went. They then came across a lightly-guarded Phantom, captured it and used its weapons to take down a large number of Covenant foot soldiers. However, the ship was soon hit and crashed near the other archaeological dig site.
While the rest of the Marines were still on their way to the site, Dutch and Romeo discovered the colony's civilians. They were being held prisoner by Covenant forces, who had used them to unearth a Forerunner complex, and were currently making the humans spring traps in the entrance to the facility. The two ODSTs took down the Covenant in the building, and avoiding the traps, made their way inside the alien complex. In the inner chamber, they encountered the Knowing, which told them that it possessed an unimaginable amount of knowledge. The ODSTs decided that the AI had to be destroyed to prevent the Covenant from gaining access to the data, against ONI's Xeno orders. Romeo told the Knowing to self-destruct, and the AI complied with his request with no hesitation. The two ODSTs escaped the underground complex just in time before the explosion. Meanwhile, the rest of the ODSTs arrived outside the dig site and noticed more Covenant forces inbound.
Following the Knowing's destruction, the Minor Prophet leading the Covenant forces declared the planet desecrated and ordered them to withdraw immediately in order to return to High Charity and inform the High Council of the incident. The Covenant retreated in a hurry, even sparing Ariel from glassing as they left.
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