Activation index

"We must collect the Index before we can activate the installation."

- 04-343 Guilty Spark

The Activation Index, referred as the Key or Sacred Icon by the Covenant, is an object found in every Halo installation's Library. The Activation Index is the key needed to activate the installation, triggering an energy pulse that will kill all sentient life within the ring's effective range. It is confirmed that an Index cannot be used to activate any Halo besides the one it is found on.

Overview
The Index is vaguely T-shaped, with a faint green or blue glow emanating from a stripe on each side. 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 extracts the Index's data and stored it within her memory, and when she activates the second Installation 04 with the stored Index. Each Index is stored safely within an Index chamber, a large room that is located at the core of every Library on each Halo installation and guarded by waves of Sentinels.

John-117 noted in his debriefing of the Halo event 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. Only one firing of the rings occurred, at the close of the Forerunner-Flood War.

It is called "The Key" by the Gravemind in his lecture to 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 Installation 00 and Installation 04B 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 Halo's 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 "Mjölnir", the hammer of the god Thor. MJOLNIR is the same name of the armor that Spartan-IIs and Spartan-III wear.
 * Cortana physically picks up the Index, although her hologram is not solid. This may have something to do with the fact that it is a computer related object, though this is never stated in-game. It is more likely that Cortana was using a computer-generated image of the Index to prove her point, or that the Forerunner's hologram projectors could actually be turned into hard light like the light bridges.
 * In the beginning cutscene of the campaign level The Library, the container holding the Activation Index unlocks, revealing it for two seconds before the animation by what is seen appears to have restarted since the Index seems to disappear as if the container was never even opened. This might have been caused by a scripting error.

List of appearances

 * Halo: Combat Evolved
 * Halo: Combat Evolved Anniversary
 * Halo 2
 * Halo 3
 * Halo: The Flood
 * Halo Evolutions - Essential Tales of the Halo Universe
 * Human Weakness