Activation index

The Activation Index, The Key or Sacred Icon as the Covenant call it, is an object found in every Halo's Library. It is the key needed to activate the Installation, causing a reaction that will purge the galaxy of all sentient life. When inserted into The Core of the Halo, the Index activates the Halo's main weapon. It is confirmed that an Index cannot be used to activate any Halo besides the one it is found on. The reason Alpha Halo's Index worked on the replacement was because they were both Installation 04.

The Index is the key to Halo's main weapon. It is needed to start the activation of Halo's final weapon sequence. Spartan-117 noted in his debriefing of the Halo incident that, if not for Cortana's intervention, he would have accidentally activated the weapon and destroyed all life in the galaxy. Halo’s main weapon exists primarily in case the Flood manage to escape containment and spread their infection to other planets and has been activated once already.

Summary
The purpose of the Halo Rings is to destroy all life in the galaxy, so that the Flood would starve to death. The Index is vaguely 'T'-shaped, with a faint green glow emanating from a stripe on each side. (In Halo 2 the index not only glows green, but it also glows a blue color.) The object seems to be merely a digital storage device containing the software or access codes needed to activate the rings, as shown when Cortana activates the replacement for Installation 04 with the Index stored in her memory. The "Sacred Icon" is first seen in Halo: Combat Evolved where, with the help of 343 Guilty Spark, SPARTAN-117 locates and attempts to activate Halo, but he is stopped just before turning the key by Cortana. Cortana then begins to argue with 343 Guilty Spark over the Index. After John-117 breaks up their argument, Cortana, explains to him the true function of Halo.

The Index appears again in Halo 2, although it is for Installation 05, when it is taken by Commander Miranda Keyes, but she and Sergeant Johnson lose it to Tartarus and his Brutes when they ambush Thel 'Vadamee. Tartarus then takes Keyes to Delta Halo's Control Room where he tries to force her to activate the ring, but fails and activates it himself (by forcing Miranda's hand down onto the Index and turning it) when Thel 'Vadamee and Johnson arrive. The Index is finally reclaimed by Miranda Keyes, who snatches it from the main platform after Thel 'Vadamee kills the Brute Chieftain.

Later, Master Chief, Thel 'Vadam and Cortana, upon discovering the new Alpha Halo, used the Index from the original Alpha Halo to activate it in order to destroy it, the Flood, and the Ark.

the Index of Installation 05 was last known to be in the possession of Commander Miranda Keyes.

It is called "The Key" by Gravemind in his lecture of the Chief and Thel 'Vadamee: "There is still time to stop the Key from turning", although with his manner of speech, it is not clear whether he was directly referring to the Index as "the Key", or just using a metaphor.

Other Purposes
The Terminals scattered within Halo 3's campaign levels entail that the Index may, apart from activating the Halos', be a record or copy of the life-forms that existed at the time of the Halos' first activation. The Librarian states that she is in the process of Indexing all life forms, and that she is close to "saving them all". This could be taken as meaning the Index contains a copy of all sentient life forms' DNA, which would, in a sense, save the individual by preserving them beyond their death. This could also explain why an Index takes time to fabricate: the DNA of all sentient life-forms may need to be re-collected for a new Index to be made, which would take much time. However, it would likely take a fraction of the original indexing time as the location of all sentient life is already known, making collection of samples rapid and efficient.

It could also be speculated that the Index must therefore be used to correctly fire the Halos' as it contains a record of all the species which Halo must target. The Monitors state that the Flood can only be stopped by starving them of sentient hosts; the Index, with its compilation of all sentient life in the galaxy, could instruct Halo to target them in the event of activation.

Trivia

 * The Index looks very similar to a symbol of Norse mythology called "Thor's Hammer" the name of Thor's hammer is Mjolnir which is the same name of the armor a Spartan wears.
 * The Halo activation order can be cancelled by removing the Index.

Related Links

 * Library
 * Halo