In the video game Halo 2, Installation 05, also known as Delta Halo (see footnotes), is one of the Halo "Fortress World" Installations built by the Forerunners to contain the Flood. It is also one of the rings that The Covenant is searching for - to control and activate them is required to complete their 'Great Journey'.

When the High Prophet of Regret fled New Mombasa, East African Protectorate (destroying the city in the process), he set up a Base of Operations on Delta Halo (Installation 05) to make the first attempt to activate it. Master Chief followed him there and assassinated him.

Delta Halo has submerged structures, ruins, and little-known wildlife. This Halo is the scene of many battles between the UNSC, the Covenant, Flood, and the Forerunner robots defending it. All three groups seek the Index and its Library is the home to the omniscient commander of the Flood, Gravemind.

When the High Prophets and the Brutes betrayed the Elites, the Brutes set up a Base of Operations in the Control Room of this Halo, in order to activate the Installation, while the High Prophets' Forerunner flagship headed for Earth in search of the Ark.

The uneasy UNSC-CS-F Alliance launched an offensive against the Brute encampment, and the Arbiter killed the High Prophets' Brute Chieftain Tartarus. The Index was retrieved, thus shutting down Delta Halo activation. However, this situation sent signals to other Halo Installations, and become ready for remote activation from a core installation known as The Ark.

Halo 3 will presumably pick up where this storyline leaves off. The fate of Delta Halo is unknown.

Its worth noting that this installation is often incorrectly refered to by gamers as 'Delta Halo', however in the game, the name 'Delta Halo' refers to a specific level set around a delta on a river (see river delta), and the ring is never referred to as this in game itself (unless you count the level description on the multiplayer map Ascension). Alternatively, this is known as Installation 05.