The Delta Halo is a ring that the Forerunners built as a device to eradicate the Flood by depriving them of flood. Essentially, the Delta Halo, like all the other rings, would send out a pulse that would eradicate all life within 25,000 light years of it. This ring could only be activated by someone human. It was first shown in the Halo series in Halo 2. In Halo 2, a brute cheiftan called Tatarus tried to activate the ring with the unwitting help of a UNSC member called Miranada Keyes, but was stopped thanks to a member of the Elites called the Aribiter and another member of the UNSC forces, Sgt. Johnson.
In the multiverse, the Delta Halo was mainly left alone, sans a few takeovers. Until the infected came, that is. The Delta Halo became their main base of operations, as Alan Schezar and Jenova found out. The Shy Guys organized a trio of task forces to try and eradicate them once and for all. With Lute, Tails, Minato, and Bao-Dur in the 3rd taskforce, designed to keep some infected away from the first 2 task forces. The second task force, involving Alan Schezar, Sparrow, Irvine Kinneas, Serina and her forces, and Shadow the Hedgehog, was designed to hold the entrance to the main control center, which housed the "erradicate all life within this galaxy" button. The first task force, trying to press this button, consisted of Amaterasu and friends, Dante, Micheal Wilson, and Vent and friends. This first group eventually fought the source of the infected himself in some strange dream world, and eventually activated the ring, eradicating the Infected from the multiverse once and for all.
Soon afterword, one of the abandoned ships of the Infected was sent on a crash course for the planet. The ship destroyed the ring, splitting it into chunks and creating the new area of space, called Pirate's Scar.
The Ruins, a rather curious part of the Delta Halo, and seemingly older than the Delta Halo itself.
The Prophet of Regret's temple, now destroyed
The Library, where the Index required to activate the Delta Halo is protected by Sentinels.
Control Room- Where the Halo can be activated
Other reports show that a large remnant of the Covenant remain on Delta Halo. This is likely due to Delta Halo's symbolic importance to the Covenant. Reports show that all races of the Covenant, minus the Prophets, are present on Delta Halo, though their numbers may have dropped sharply due to the Infected incident.
Though unconfirmed, it is largely believed that the AI constructs known as 'Sentinels' still protect the ring.
A powerful parasitic race known as The Flood also thrive on Delta Halo. Shy Guy Reports suggest that, like the Infected, the Flood inhabit Halo because of a Hive-Mind with a powerful flood deity known as 'Gravemind'. This Gravemind has not been spotted by anyone, so whether it still lives or not, is unknown.
The remaining Covenant forces will protect this world zealously. Prepare to fight their advanced technology and highly-trained soldiers to the death.
The Flood will attack anyone on Delta Halo to add to their numbers.
Theories suggest that the Sentinels will attack anyone who comes to the ring for violent means. If you simply crash-landed on the ring and wish to leave, they will leave you be, possibly assisting you. However, if you come to destroy anything on the ring, or seek to find treasures in the ruins, expect a fight with these moderately powerful machines.