Keystone

A Keystone is a Forerunner artifact that holds Forerunner information.

Overview
At least one Keystone was buried on the planet Madrigal. The Keystone could display a holographic Forerunner script, which led to at least one Halo ring. Furthermore, the Keystone has shown the ability to mass EMP and repower UNSC equipment. When touched, the Keystone is able to scan the person who touches it, and, in one instance, awakened locked memories in John-117.

Madrigal Keystone
In 2552, the Covenant launched an attack on Madrigal for the keystone, wiping out a nearby Insurrectionist outpost. The Spartans of Silver Team eliminated the attacking Sangheili and John-117 and Kai-125 entered the cave where the Covenant had been excavating for the artifact. Upon finding the keystone, John touched it and the artifact reacted to the touch of a Reclaimer, lighting up the entire cave and displaying symbols. It had a surprising effect on John, apparently unlocking some of his sealed memories of his childhood. The artifact's reaction to John was noticed by a surviving Sangheili who managed to escape and report it back to the Prophet of Mercy who had learned of the artifact's location from Makee.

Throughout his return trip to Reach, John remained fascinated by the artifact while Dr. Catherine Halsey became concerned about its effects on John and his memories. After John rebelled rather than execute Kwan Ha, the UNSC shot down his Condor with an electromagnetic pulse, knocking the artifact loose from its casing in the process. John touched it, causing the symbols to display once again and for John to experience more of his memories. In one of his memories, a young John drew a picture of the artifact for his mother. The artifact then let out a pulse that knocked out power to the UNSC base while restoring power to John's Condor, allowing John to escape from Reach with Kwan and the artifact.

John subsequently took the artifact to the Rubble to seek out Soren's help. Soren's touch failed to elicit a reaction from the artifact and he introduced John to Reth, a man who had been on one of the Covenant ships and had been tested as a Reclaimer. Like with Soren, the artifact also failed to respond to Reth who explained that the ring that it displayed when activated was a weapon of great power that the Covenant must not find and he urged John to destroy it and then himself. When Reth tossed the artifact at John, he caught it, triggering a pulse that caused everyone but John great pain and for the symbols to display once again as well as causing John to see more of his childhood memories. John then decided to take the artifact back to the UNSC where it was studied by Doctor Catherine Halsey.

At the same time, on High Charity, Makee and the three Covenant Hierarchs listened to the surviving Sangheili warrior's testimony about what he had seen. Alarmed that the Demon had gotten his hands on one of the keystones, Makee decided to pursue it herself.

Production notes
The Madrigal Keystone uses a form of Forerunner symbols featured in the core canon, with the relic in Contact saying "Scan - End".

List of appearances

 * Halo: The Television Series
 * Contact
 * Unbound
 * Emergence
 * Homecoming