Holocrons

This image is to help explain the state of affairs for my Star Wars Fanfiction Series: Star Wars: The Shadow Wars. The series is currently between "Seasons."

Note: The information below is from my own universe which is a blend of old and new canon and it not to be considered canon in any other universe but mine.

The image was acquired through Garry's Mod

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Holocrons
For millennia, the Jedi and Sith have relied upon holocrons to record knowledge and information and preserve them for generations. The technology used to create holocrons are more complex than an ordinary holo-recorder though as, with a few exceptions, the holocrons record the cognitive processes of the person who created them, making the AI personalities within holocrons semi-sentient and able to detect the skill and ability of its user. These personalities are referred to as "Gatekeepers." Such technology made sure that Padawans could not access forbidden knowledge or information that was above their skill level.

This technology was sometimes used to create Gatekeepers for instances when a holocron was not used. One such recorded case was the casket of Ulic Quel-Droma. Ulic's tomb on Rhen Var was equipped with a gatekeeper that held the memories and experiences of the late Jedi Master for almost 4000 years and would impart the knowledge needed to defeat the ancient Sith Superweapon, the Dark Reaper, on to Jedi Padawan Anakin Skywalker during the Clone Wars. This same Gatekeeper was later destroyed almost 50 years later by the Sith to try and prevent this same knowledge from being retrieved by the New Jedi Order but Skywalker, still remembering the knowledge from Quel-Droma's tomb, had already passed this knowledge along to each of his apprentices.

During the Great Jedi Purge, Emperor Palpatine stole the Jedi Holocrons from the Jedi Temple. He was tempted to destroy them to prevent future generations of Jedi, should they arise, from being able to access them. However he believed there might be valuable information he could exploit on the holocrons and preserved them, hiding them away within a hidden vault on Pillio. Shortly after the Emperor was killed at the Battle of Endor, the vault on Pillio was on a list of targets for destruction by the Emperor in the event of his death, to prevent the Jedi knowledge from falling back into Jedi hands. However, Del Meeko, the Imperial Agent tasked with demoing the vault abandoned his mission after being rescued by Jedi Knight Luke Skywalker, permitting the Jedi to retrieve whatever they wanted from the vault before he destroyed it. The vast library of Jedi Holocrons were recovered from the vault and stashed away in a secure location until Luke Skywalker finally established his New Jedi Order on Yavin IV, Jedi Master Tionne Solusar being placed in charge of the Holocron Vault. The Jedi managed to successfully rescue all of their holocrons from Yavin IV during the Sith attack in 28 ABY and again rescued their Holocrons during the evacuation of Ossus a year later, returning the holocrons to a new vault within the Jedi Archives of the Jedi Grand Temple on Coruscant.

The only holocron available to the general Jedi Population is the Great Holocron, which houses information entered by countless Jedi from across numerous eras and can be accessed by even Jedi Initiates. Even a few members of the Jedi Temple Guard have been able to access the Great Holocron. All other holocrons are kept inside of a holocron vault within the Jedi Temple. 

Excluding the Great Holocron, the New Jedi Order has divided Holocrons into two categories: Specialized and Generic. Specalized holocrons are holocrons made by a specific Jedi to house their knowledge and experiences and typically have Gatekeepers made from their own personalities. Generic Holocrons typically hold pieces of information from numerous different Jedi and don't necessarily have a Gatekeeper equipped, most Generic Holocrons being able to sense what information the user wishes to access. One such holocron is the one carried by the Jedi Kanan Jerrus which houses a series of instructional recordings from Jedi such as Anakin Skywalker, Mace Windu, Yoda, and several others as well as a warning message recorded by Obi Wan Kenobi shortly following Order 66. These Generic Holocrons can also read information such as the information stored on a Kyber Memory crystal.

Some of the Jedi known to have created Specialized Holocrons include but are not limited to...
Bastila Shan
Arca Jeth
Nomi Sunrider
Vima Sunrider
Ulic Quel-Droma (creating both a Holocron and a Gatekeeper on Rhen Var, the Rhen Var Gatekeeper was the more recent of the two)
Revan*
Meetra Surik*
Satele Shan
several others...

The Sith have also created holocrons of their own. However, the Sith have almost exclusively created Specialized Holocrons, likely as a subtle show of ego and arrogance or desperation, with only a handful of known Sith Holocrons being of the Generic type.

Some of these Sith include but are not limited to...
Darth Bane
Darth Nihilus
Darth Andeddu
Darth Revan*
Darth Adas
Darth Sion
Darth Malgus
Darth Malak
Darth Magnus
Darth Plagueis (rumored, unconfirmed)
Darth Tyranus/Count Dooku
Darth Sideous/Emperor Palpatine
and several others.

The Jedi have been unable to reclaim most if any of the Sith Holocrons and these holocrons are believed to be in the hands of the Sith Empire.

It's interesting to note that Revan is one of the few individuals to have made both a Jedi and a Sith Holocron. Revan's Jedi Holocron is the newer of the two and is unusual as it houses two Gatekeepers instead of one. Revan's gatekeeper is the predominant gatekeeper within the holocron while the gatekeeper of Meetra Surik, commonly known as the Jedi Exile, will activate to recount her own experiences. Revan's Jedi Holocron is believed to have been made prior to Revan and Surik's mission to confront the Sith Emperor Vitiate almost 4000 years before Yavin. Revan's Sith Holocron, made during the height of the Jedi Civil War, is believed to also be in the hands of the Sith Empire under Darth Akul.

Of the various Sith Holocrons, an unusual one exists. This holocron, created by a self proclaimed Sith Lord known as Darth Magnus, was created during the Jedi Civil War and would help to guide Dark Jedi to Darth Magnus's hiding place where an army of Sith Soldiers from the Jedi Civil War along with Magnus, were frozen in Carbonite. It was the Magnus Holocron that was stolen from the Hutts by the crime lord Tyber Zann and used by the Nightsister Silri in 4 ABY to find this army. The army itself was nothing more than mindless drones, their minds warped from almost 4000 years of hibernation sickness, including Magnus, but the secretive Sith Empire was able to put these soldiers to use as templates for its own cloned army.

It's also been confirmed by the recently defected Talzin Kenobi, that Darth Sideous/Emperor Palpatine created a Holocron for Darth Akul to study in his absence and is one of the few Sith Holocrons that none of the other Sith of the Sith Empire are allowed to access as the Empress keeps it secured away in her private chambers. Count Dooku is also revealed to have made a Sith Holocron during the Clone Wars as a means of tutoring prospective Sith Acolytes and Dark Jedi. Talzin has reported that Tyranus's Holocron is made readily available to Sith Prospects and covers some of the basic Sith abilities such as Force Choke and Force Lightning, as well as recounting some of Dooku's own experiences in his fall to the Dark Side. It should be noted that non-Jedi holocrons exist. These Data Holocrons are used mainly to store encrypted data or serve as interactive databases via their Gatekeepers.

Counterfeit holocrons also exist, especially by groups and individuals attempting to cash in on the Jedi and the Sith's mysticism. These counterfeit holocrons can range from simple broken holoprojectors to elaborately designed devices with recordings of individuals claiming to have been ancient Jedi or Sith. It's rumored that an Ithorian on the planet Batuu sells replicas of Jedi and Sith Holocrons, supposedly housing ancient Jedi and Sith Teachings. These holocrons have been confirmed to be (comparatively) cheap replicas.