Raid on the Threshold gas mine

The Battle of the Gas Mine took place during the Ninth Age of Reclamation between the forces of the Covenant, under the command of The Arbiter and SpecOps Commander Rtas 'Vadumee and the Heretics, led by Sesa 'Refumee. The SpecOps force's objective was to assassinate 'Refumee, whom the Hierarchs saw as a threat and offence to the Covenant.

Sentinel forces on the installation apparently sided with the Heretic forces, presumably due to 343 Guilty Spark's presence there.

The Flood later joined the battle, possibly due to the carelessness or design of the Heretics.

Sequence of Events
Sometime after the Battle of Installation 04, the Monitor 343 Guilty Spark arrived at a Forerunner Gas Mine on Threshold. There, he met a Covenant search team, lead by Sesa 'Refumee. 'Refumee, learning of the truth of the Halo Rings, inspired rebellion and led his team in a Heretic Campaign.

His team made their base at the mine, fortifying it with shielded turrets, Banshee aircraft, and Anti Aircraft Fuel Rod Guns. In defiance of the Covenant, they dressed in the armor originally worn by the Elites on their homeworld, and armed themselves with Needlers, Carbines, Fuel Rod Guns, Plasma Rifles and Minor Sentinel Beams.

343 Guilty Spark reinforced these defenses with Sentinels and intelligence. Here, the Heretics lashed out at any Covenant assault team that came near.

Around this time, the disgraced Supreme Commander was given the Prophets' blessing and led a SpecOps team against the Heretics, as the Arbiter.

After entering the structure, the Arbiter and his team met with minor resistance from the Heretic soldiers.

After battling their way to the hangar bay, they encountered Sentinels that were allied with the Heretics.

After clearing the landing bay, and dealing with Heretic reinforcements, the Arbiter and the SpecOps team ventured further into the facility.

Later on, after heavy resistance, the Arbiter was forced to proceed by Banshee. He and a Phantom cleared the outside of the facility of enemy soldiers and Banshees, and then the Arbiter and another squad of SpecOps Elites entered the center of the facility.

When they entered the structure, they encountered the Flood. After an elevator ride, on which they battled both the Flood and numerous Sentinels, and fighting their way through numerous Flood-infested corridors and rooms, the Arbiter encountered Sesa 'Refumee.

'Refumee escaped, however, and the rest of the SpecOps Elites left while the Arbiter continued upward to cut the cables holding that section to the rest of the facility, which, according to 343 Guilty Spark, was a fail-safe technique to be used in case the Flood ever escaped containment. Successfully cutting the cables, he chased the Heretic leader to his Banshee and followed him to back to the hangar bay.

The Arbiter fights his way past numerous groups of Heretics and Flood and reaches the Seraph fighter, which 'Refumee is about to enter.

343 Guilty Spark appears but, before the Arbiter can question him, 'Refumee attacks.

After a brief battle, the Arbiter drags the dead Heretic's dead body to 343 Guilty Spark.

After a brief exchange of words, the monitor is captured by Tartarus. The Arbiter then leaves the Gas Mine Facility on Tartarus' Phantom.

Outcome
The Arbiter managed to kill the Heretic Leader. 343 Guilty Spark was captured by Tartarus and taken before the High Council. All the Heretics died at the hands of the Flood. The Flood was released, but did not escape the mine itself. Any surviving Heretics that didn't die to the Flood were most definitely killed when the station fell deeper into Threshold's atmosphere after the Arbiter cut the main cable. All the Flood left on the station presumably died as well.

In the Game
The Battle of the Gas Mine covers two levels in Halo 2, The Arbiter and The Oracle. These two levels are the first ones played as The Arbiter.