The Index is an item that comes up in both Halo 1 and Halo 2. Each of the Halo Ringworlds are beacons of sorts. They are not active though, so the beacon needs to be activated at the installation's control room before the beacon can fire and set off the ultimate Halo weapon.
In Halo: Combat Evolved, the Master Chief and The Monitor have to brave the Flood-infested Library to retrieve the Index to activate Installation 04. After the most infamous level ever in the Halo Trilogy, you retrieve the Index from the heart of the Library, then you have to take it out of the Library and directly to the Installation's Control Room to activate Halo. Of course, this is before the Master Chief is aware that activating the Installation will destroy the universe.
In Halo 2, it is the Arbiter who must retrieve Installation 05's Activation Index. He does so by visiting Installation 05's control room, much the same way as Master Chief did in Halo: Combat Evolved, only this time, it's for the purpose of making sure that the Brutes cannot activate Installation 05 with it.
The Story culminates in an epic battle between The Chieftan Brute, Tartarus and The Arbiter in the control room of Installation 05. Ultimately, the Arbiter wins and Installation 05 is saved by Miranda Keyes as she snatches the Index from Installation 05's core and destroy it with Tartarus own Gravity Hammer.
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