From Halopedia, the Halo wiki
Black Reef was an island hosting a large mining facility that was home to hundreds of colonists, and oversaw a massive and sprawling system of caves beneath the surface. It was a tropical settlement home to many beaches, among them Bondi Beach. By 2525, the United Rebel Front had established its base of operations here, under the command of Captain Whitaker. By the time of the Covenant invasion, the mining facility had been heavily fortified by both infantry and rebel-operated naval vessels.
On December 3, 2525, a large Covenant force invaded Alpha Corvi II, deploying thousands of ground forces to the capital city of Jamshid while also sending an expeditionary force to Black Reef via dropship. Upon landfall, the URF forces under Whitaker's command immediately rushed to the base's defense, where the Covenant slaughtered the entire civilian population and any rebels who stood in their way. While the URF fought to repel the invaders, the Covenant sent nearly a hundred infantry into the island's vast cave system in search of an unknown Forerunner artifact. During this, Spartan-II supersoldier John-117 of Blue Team introduced himself to Captain Whitaker and informed him of the severity of the threat posed by the Covenant, convincing the rebels to align themselves with the Spartans to retake the island.
Whitaker and his men employed a heavily-armed APC to plow through the Covenant infantry, and bought Blue Team the time to enter the mining shaft where the Covenant were already deployed. While Blue Team fought the Covenant underground, Whitaker and his forces fought off multiple waves of Covenant reinforcements. Unable to stop their advance, Whitaker and his men destroyed the mine entrance to deny the enemy the advantage they sought. Whitaker then took the APC along with several soldiers to rendezvous with the Spartans on Bondi Beach. The Spartans meanwhile were forced to swim their way up back to the surface as the caves around them had been collapsed by the blast. After wiping out the Covenant expeditionary force and denying them access to the artifact, Blue Team linked up with Whitaker's forces on the beach, where the Covenant had already laid waste to the human fortifications present. Pelican dropship Tango 807 then arrived to evacuate the Spartans, and both sides quickly cleared the site for the ship to land. While Blue Team was safely evacuated, the rebels were abandoned by the UNSC against John-117's protests, and Whitaker was killed along with the rest of the rebels when Covenant reinforcements overran the beach.
Black Reef was likely glassed along with the rest of the planet as the UNSC was forced to retreat from the Alpha Corvi system.