Delta Halo was the second Halo installation discovered. It was revealed by 343 Guilty Spark to the Covenant High Prophets after they found him in the Heretic base above Threshold. The Heretics had found him floating in the wreckage of Alpha Halo, and he had revealed the true purpose of Halo to them, which had caused them to abandon the Covenant.
High Charity jumped in system while the Arbiter and Covenant troops scoured the Ring for an Index, which they could use to activate the installation. Unfortunately, the ring had suffered a fairly widespread Flood outbreak, which made reaching the Library quite a challenge for the Arbiter. However, the Arbiter did succeed in retrieving the Index, but immediately after he was thrown into a pit by Tartarus, where he encountered the Gravemind, the compound mind of the Flood.
This Gravemind used the Arbiter and the Master Chief to stop Truth from activating rings. This was simply a distraction from the Gravemind's true intentions of escaping Delta Halo with captured Pelican drop ships. These Flood succeeded in infecting High Charity, but the Covenant fleet in orbit destroyed most of the ships trying to escape the ring. It is possible that Delta Halo was completely glassed by the Elite Separatist fleet to prevent the Flood infection from further spreading.