Raid on the Threshold gas mine

The Battle of the Gas Mine took place during the Ninth Age of Reclamation.

It was an attempt by Covenant SpecOps forces, under the command of The Arbiter and Rtas 'Vadumee, to eliminate the Heretic leader, Sesa 'Refumee.

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 of the Heretics.

All Flood and Heretic forces (excluding Guilty Spark) are presumed destroyed, due to the destruction of the Gas Mine Facility.

Sequence of Events
Sometime after the Battle of Installation 04, the Monitor (343 Guilty Spark) arrived at a Foreunner Gas Mine.

There, he met the 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 holed up at the mine, fortifying it with shielded turrets, Banshee aircraft, and Anti Aircraft Fuel Rod Guns.

They dressed in the armor of the Elites' 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 escapes, however, and the rest of the SpecOps Elites leave, while the Arbiter continues upwards to cut the cables holding that section to the rest of the facility, which, according to 343 Guilty Spark, is a fail-safe technique to be used in case the Flood ever escape containment.

After the Arbiter cuts the cables, he chases the Heretic leader to where the Banshees are.

He is alerted by Rtas 'Vadumee that Sesa will attempt to escape in the Seraph that was in the launch bay the Arbiter had received reinforcements from.

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, which was their mission objective. 343 Guilty Spark was captured by Tartarus and taken to the High Council. All the Heretics died at the hands of the Flood. The Flood was released, but didn't escape the mine itself. Any surviving Heretics that didn't die to the Flood and any Flood were most definitely killed when the station fell deeper into Threshold's atmosphere after the Arbiter cut the main cable.

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.