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. Its secondary purpose is to simultaneously initiate the Conservation Measure, the repopulation process of the worlds sterilized by the Halo's pulse. 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 extracted the Index's data and stored it within her memory, and when she activated 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.

When inserted into the Core inside a Halo's Control Room, the Index immediately initiates the charging sequence of the ring's main weapon. The activation process may be aborted by physically removing the Index before the firing; however, doing so sends out a signal which places the entire Array into standby mode, ready for activation from the Ark. An AI construct based in the Core is able to prevent the initiation of the ring's firing sequence even if the Index is inserted, as demonstrated when Cortana prevented John-117 from activating Installation 04.

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. Immediately following the activation, the secondary purpose of the Index also came into play, triggering the re-seeding of life on the worlds whose sentient species were indexed in the records of each Installation's Library in addition to the archives on the Ark.

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.

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 some Spartan-III wear.
 * 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