Background info
One of the few four-sided battles of the 9th Age of Reclamation, the Covenant and UNSC fought through the Quarantine Zone to look for the Index, called the Sacred Icon by the Covenant. Both sides had to battle their way through massive numbers of Flood and Forerunner machines: Sentinels and a newly introduced type of Sentinel, Enforcers.
Battle
The Arbiter was sent to the surface of Delta Halo to retrieve the Sacred Icon.The Arbiters main objective was to lower the containment sheilds to allow more Covenant warriors to enter the Quarantined Zone. After he lowered the Containment shield, the Flood were allowed to roam freely along Delta Halo. The Humans of In Amber Clad that were searching for the Index were surprised by this and most of them were assimilated by the Flood, enlarging their numbers. The new Human Flood learned to use their vehicles as well: Scorpions and Warthogs. The Covenant warriors who were sent to retrieve the Index were assimilated as well by the Flood. With this, the Flood had even more vehicles: Wraiths and Ghosts. Despite this new and surprising defense, the Arbiter and his squad of SpecOps Elites were able to battle their way trough the countless Flood and Sentinels. After a series of battles the Arbiter regrouped with Rtas 'Vadumee and followed the Humans with a gondola that was headed towards the Library. Rtas 'Vadumee stayed didn't board the gondola, and he stayed behind to keep the flood away from the gondola. Along the ride, the Arbiter and the squad of SpecOps Elites were attacked by numerous waves of flood.
Library
Once the Arbiter and his squad arrived at the Library, the Humans were already inside. The Arbiters squad of Elites were left behind to guard the gondola since it was their only way out. The Arbiter found the Humans (Miranda Keyes, and Sergeant Johnson) who had retrieved the Index seconds before the Arbiter arrived. The Arbiter knocked out Johnson but, before he could get to Miranda, Tartarus showed up along with a squad of brutes and, using his Gravity Hammer, knocked out Miranda and took the index from her. He then blasted the Arbiter off the edge of the platform where the Index was stored, leading Tartarus to make the assumption that he was dead.