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.

Prelude
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.

The High Prophets of Truth and Mercy viewed these Heretics as a threat to the Covenant and an offence to their religion. Against the wishes of the High Council, Truth promoted the disgraced former Supreme Commander of the Fleet of Particular Justice to the rank of Arbiter, and sent him and a team of SpecOps Commandos to destroy the Heretics.

The Battle
With a heavy storm masking their approach, the team silently entered the structure with the element of surprise, encountering only minor resistance from the Heretic soldiers. After battling their way to the hangar bay, they discovered that, to their great surprise, that the Mine's Sentinels were allied with the Heretics. After clearing the landing bay, and dealing with Heretic reinforcements, Rtas 'Vadumee sent in the second Lance of Covenant troops, and 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 a hellish elevator ride, on which they battled both the Flood and numerous Sentinels and resulted in the deaths of many Covenant soldiers, 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 departed 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 fought his way past numerous groups of Heretics and Flood and reached a Seraph fighter that would serve as 'Refumee's escape craft. The Arbiter ordered the Heretic to surrender, but was distracted by the arrival of 343 Guilty Spark. Before the Arbiter could question him, 'Refumee attacked the distracted Elite. Despite having to fight past numerous holograms deployed by 'Refumee, the Arbiter killed the Heretic leader.

Outcome
After 'Refumee's death, 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.