Activation index (Earth-2915)

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

- 343 Guilty Spark

The activation index, often simply referred to as the index and known as the sacred icon by the Covenant, is an object found in every Halo installation's Library. The activation index is the key needed to operate the installation's most critical systems, the most notable of which is its main weapon, 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. An index cannot be used to activate any Halo besides the one to which it is assigned.

Source

 * https://www.halopedia.org/Activation_index