The Index or Sacred Icon is an object found on every Halo. It is the key needed to activate the Installation, causing a reaction that will purge the galaxy of sentient life. When inserted into The Core of the Halo, the Index activates Halo's main weapon. It is not known if an Index can be used to activate any Halo besides the one it is found on.
The Index is the key to Halo's main weapon, whatever that might be. 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. Current theories suggest Halo’s main weapon exists primarily in case the Flood manage to escape containment and spread their infection to other planets.
Summary
The purpose of the destruction of all sentient life is to starve The Flood, who cannot be stopped otherwise. It is roughly T-shaped, with a faint blue-green glow emanating from a stripe on each side. It also seems to be able to be stored in a network, meaning that it is made out of an advanced material. Halo cannot be activated unless the Index is inserted into the control panel in Halo's control room. The "Sacred Icon" is first seen in Halo: Combat Evolved where (with the help of 343 Guilty Spark) the Master Chief locates and attempts to activate the Halo, but he is stopped just before turning the key by Cortana. Cortana then begins to argue with Guilty Spark over the Index. After the Master Chief breaks up their fight, Cortana explained to him the true function of Halo, 343 thought that the Master Chief already knew that, and later he states to the Chief: “Last time you asked me, if it was my choice, would I do it? Having had considerable time to ponder your query, my answer has not changed. There is no choice. We must activate the ring." and later: “Why would you hesitate to do what you’ve already done?”.The Index of Installation 04 was last known to be in the possession of Cortana. The Index of Installation 05 was last known to be in the possession of Commander Miranda Keyes.