Installation 08

The Replacement Installation 04 was a Halo partially constructed and completed by the Ark after the destruction of Installation 04. This ring was designed to be nearly identical to Installation 04, and to replace it in the Halo network, once again completing the Forerunner's plan for seven Halo rings spread throughout the galaxy with the ability to cleanse all sentient life, thus protecting the galaxy itself from the threat of the Flood.

History
In Halo 3, Installation 04 is rebuilt, by an automatic function in The Ark that replaces a Halo when it is destroyed. When the new Installation 04 was released into space it was incomplete. John-117 and the Arbiter decided to activate the incomplete installation to eradicate the Flood army that was rapidly spreading across the Ark. Guilty Spark, who had arrived and allied with the two, expected such a recreation, and became almost ecstatic at the thought of finally activating the ring.

The Master Chief and the Arbiter were forced to make a detour to, the now long consumed, High Charity, in order to recover Cortana, and the activation Index from the original Halo. John then detonated the city's damaged plasma reactor to destroy the hive and the Flood Gravemind in order to slow the rapidly growing legions of Flood.

John, Cortana and the Arbiter landed in the snowy canyon that housed the Core in order to activate the ring. After eliminating the area's surviving Flood, who were trying to rebuild the Gravemind on the ring, the three meet with Sergeant Johnson and enter the control room.

Guilty Spark informs Johnson that the ring will require several more days to become fully operational, and that a premature firing will destroy both the new ring and the ark. Due to the gravity of the situation, Johnson disregards Spark's warnings and prepares to download Cortana into the Core. Spark, outraged by Johnson's reckless behavior, fully submits to his insanity, and attacks him, mortally wounding him. John and the Arbiter attempt to stop the insane construct, but are both subdued by Spark's energy weapon. He then goes on to lecture the Chief on how even though he is descended from his creators, "...this ring is mine."

SPARTAN-117 then battles with Spark, but finds that he is unable to penetrate the monitor's shielding. With the last of his strength, Johnson attacks 343GS with his Spartan Laser, damaging the neurotic monitor's casing, thus allowing John to destroy him. Johnson hands the Spartan Cortana's data chip, pleading the Master Chief to leave him and activate the ring, before dying. Cortana broadcasts herself to the console of the control room, beginning the activation sequence.

As Guilty Spark predicted, the ring was not ready to fire, and began it disintegrate. John and the Arbiter escaped through the first Phase Pulse Generator room, and then to Johnson's Warthog. The vibration of the activation began to destroy the ring, causing explosive reactions across the installation. John and the Arbiter barely managed to reach the UNSC Frigate, UNSC Forward Unto Dawn, before the ring detonates and "[does] a number on The Ark", according to Cortana.