Keystone

A Keystone is a Forerunner artifact that holds Forerunner information.

Overview
At least one keystone was buried on the planet Madrigal with another keystone that went together with it on Eridanus II. 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 awakened locked memories in John-117. However, each time that John used the keystone it placed a strain on his heart and nervous system.

When the Madrigal keystone was activated and John was put into stasis by Cortana so that the AI could access it herself, the artifact immediately shut itself down, seemingly needing John to be conscious to work. Later, after removing his emotional regulator, John realized that his altered hormonal state might've been affecting his interactions with the keystone. Following the regulator's removal and the restoration of a normal hormonal state, John's unlocked memories became clearer to him and easier to understand.

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 keystone to the Rubble to seek out Soren's help. Soren's touch failed to elicit a reaction from the keystone 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. When Reth shoved the keystone into John's hand, it triggered 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 quickly returned the keystone to its box, ending the effects. Reth explained that the ring that it had displayed when activated was what the Covenant and a door to the end of life as they knew it. Reth confirmed that the ring was a weapon and after John admitted that he could feel the darkness, Reth urged John to destroy the keystone and then himself. However, John then decided to take the artifact back to the UNSC where it was studied by Halsey who described it as being nothing like she'd ever seen before.

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. Makee attacked the with the help of a Lekgolo colony, but they failed to get any information from either the crew or the ship's database which was wiped during the attack as per the Cole Protocol.

Admiral Margaret Parangosky later told Miranda Keyes that results confirmed that the keystone was over 100,000 years old which Miranda was stunned by, calling it pre-human and possibly even pre-Covenant. Miranda noted that the keystone was organic, but with what appeared to be an inherent technology. Parangosky revealed what John had learned about it leading to a weapon somewhere and asked Miranda to lead a team analyzing the artifact to determine what it was, what it did, if it was in fact a part of some kind of a weapon or anything that could give humanity an edge over the Covenant. Parangosky sought to create parallel teams studying in the artifact in order to find a way to be less reliant on Halsey in the future.

Under monitoring by Halsey and Adun Saly, John activated the keystone once again, triggering more of his childhood memories featuring a young John drawing and exploring a glowing cavern. As John reached for something in the memory, Halsey had Cortana put John into stasis so as to have Cortana access the artifact herself. However, as soon as Cortana shut John down, the keystone deactivated itself and Cortana got no readings from it at all. Halsey had Cortana release John who called this experience with the artifact "different" than the previous times. With Cortana's help, John later attempted to locate the planet from his memories using the FLEETCOM Central Database.

After removing his emotional regulator and having an epiphany after seeing a dog in the park, John returned to the artifact and had Cortana, with Halsey's consent, override the lock on the room's door. Despite Cortana's warnings that touching the keystone put a strain on his heart and nervous system each time, John activated it once again and unlocked more of his childhood memories which revealed that he drew both the Madrigal keystone and another larger one, showing his mother that the two artifacts went together and that John's father had eventually forced him to bury something against his will. With the added information from his newly-unlocked memories, John identified the planet that the second keystone was on as Eridanus II. John subsequently explained to Halsey that John had realized that his altered hormonal state from the regulator had been messing with his interactions with the keystone.

Eridanus II Keystone
A second keystone existed on Eridanus II, one that went with the Madrigal keystone. It was discovered by John-117 as a child, causing him to repeatedly draw both the Eridanus keystone and the Madrigal one, recognizing that they went together. Eventually, John's father grew frustrated and forced John to bury his drawings of the keystones.

In 2552, after the adult John touched the Madrigal keystone, his blocked childhood memories surrounding the Eridanus keystone started to be unlocked. John eventually remembered that there was a second artifact on his home planet and located the correct world with Cortana's help, although Eridanus II was uninhabited by this point and sealed off, most of the population having been wiped out by a plague. John decided to return to Eridanus II in search of the keystone and Dr. Catherine Halsey insisted upon going with him. Cortana was able to generate a holographic image of the Eridanus keystone based on John's description for Halsey and Adun Saly.

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