Installation 05 was a Halo ring orbiting Substance. It was monitored by 2401 Penitent Tangent. It was a jungle-based ring that suffered a major Flood outbreak during the 100,000 year wait for Reclaimers to arrive.
The Flood spread throughout the ring and 2401 Penitent Tangent was captured. After waiting under the Installation's Library for thousands of years the Gravemind formulated a plan for his ascent. (Note: close inspection of the Library reveals that there are no books. This is because Gravemind ate them all to use for his poetry.)
Thousands of years after the outbreak, Johnny and Cortana landed on the ring in the In Amber Clad. With Miranda Keyes providing them with intelligance they snuck through the forests of the ring (and its many ruins) and eventually assassinated the Prophet of Regret.
Horrified, the rest of the Covenant sped to the ring but were too late to save the Prophet, and so his temple was destroyed instead. This was all a ruse by Truth, however, as he wanted Regret out of the way. Tartarus then tried to activate this ring, but was stopped by Guilty Guilty, Sergeant Johnson and the Arbiter.