The heretics

Originally, the Heretics were a Covenant artifact retrieval team attached to the Fleet of Particular Justice and led by Sesa 'Refumee. They were investigating a Gas Mine on the planet Threshold when the UNSC Pillar of Autumn crashed on Halo. When the Master Chief destroyed Halo, the team was spared from death.
A few days later, the retrieval team located the Monitor of Installation 04, 343 Guilty Spark, floating adrift in the wreckage of Halo. Spark promptly took command of the Sentinels running the gas mine and cheerfully informed the artifact retrieval team of the true purpose of the Halo Array, utterly destroying their faith in the promised "Great Journey."
Disillusioned with the lies of the Prophets and their Great Journey, 'Refumee initiated an open rebellion against the Prophet leadership and the basis of the Covenant. In secret, he sent holographic recordings to other Sangheili, hoping to open their eyes as well. In the meantime, he strengthened the defenses of the gas mine, fortifying it with Plasma Cannons and a large compliment of Banshee fighter craft in anticipation of the Covenant's retribution for heresy. He also had at least one Seraph fighter, as seen in the docking bay.
It is unclear how many more Elites 'Refumee was able to sway to his cause, but one of the Sangheili he attempted to bring to his cause instead turned the holo-recording over to the High Prophets, who decided to put the "Heretics" down with two other former commanders of the Fleet of Particular Justice: the disgraced Thel 'Vadamee, now the Arbiter of the Covenant, and Spec-Ops Commander Rtas 'Vadumee.
Whilst inhabiting the mine, the Heretics discovered that the facility had a secondary purpose: a Flood Research Facility was built into the mine as part of the Forerunners' attempt to find a way to destroy the Flood. The Arbiter and 'Vadumee arrived at the facility with orders to annihilate all Heretics. Accompanying them were three Phantoms full of Spec-Ops Sangheili and Unggoy, fighting through a large contingent of Heretic soldiers whilst chasing 'Refumee throughout the facility.
Whether by accident or design, the Heretics released the Flood upon themselves shortly before the Arbiter's arrival - creating a deadly triangle of conflict as all sides attempted to destroy the others. Despite heavy resistance from the Heretics and the Flood, the Arbiter was able to kill Sesa 'Refumee and destroy the gas mine completely by severing the support cables maintaining the facility's position.I have got this info from halopedia.com