Cyberium

The Cyberium was an AI which contained the knowledge and future history of all Cybermen, (TV: The Haunting of Villa Diodati) serving a role once held by the history computer. (TV: The Tomb of the Cybermen, PROSE: Iceberg, et al.)

Initially used by the Cybermen (TV: The Haunting of Villa Diodati) during the Cyber-Wars of an era that the Thirteenth Doctor would describe as "the very far future", (TV: Ascension of the Cybermen) the Cyberium was eventually acquired by a human resistance force known as the Alliance, who sent it back in time (TV: Fugitive of the Judoon) to Earth in 1816, where Percy Shelley became its "guardian". After Ashad, the Lone Cyberman, forced the Thirteenth Doctor to relinquish the Cyberium or allow the Earth to be annihilated, Ashad travelled with the Cyberium (TV: The Haunting of Villa Diodati) to the immediate aftermath of the Cyber-Wars, where he used the Cyberium's knowledge in an attempt to resurrect the Cyber-Empire (TV: Ascension of the Cybermen) and destroy all organic life. Following a failed alliance between Ashad and, the latter became the Cyberium's new host and created the CyberMasters. (TV: The Timeless Children)

The Cyber-Wars and the Alliance
During the Cyber-Wars of an era that the Thirteenth Doctor would describe as "the very far future", the Cybermen used the Cyberium as a highly advanced strategist; the Doctor would describe the Cyber-Wars as "an epic battle", with the Cyberium "at the heart of it, controlling data, strategy [and] decision making". (TV: The Haunting of Villa Diodati)

Sometime either during the Cyber-Wars or in the immediate aftermath of the conflicts, (TV: Ascension of the Cybermen) the Alliance sent the Cyberium back through time and space in order to prevent the return of the Cyber-Empire. In the 21st century, Captain Jack Harkness informed the Thirteenth Doctor's companions, Graham O'Brien, Yasmin Khan and Ryan Sinclair, about this, but did not specify exactly what the Alliance had sent back, though he did tell the trio to send the Thirteenth Doctor a warning, telling her to "beware [of] the Lone Cyberman" and not to "give it what it [wanted]". (TV: Fugitive of the Judoon)

Percy Shelley, the guardian
While out walking in June 1816, Percy Shelley spotted the Cyberium in Lake Geneva. When he fished it out of the water, it melted in his hand and entered his bloodstream. Upon returning to Villa Diodati, the Cyberium tried to hide itself by making Shelley invisible to everyone else and even utilized a perception filter to keep from being discovered. Meanwhile, Shelley acted as its guardian. At the same time, the Thirteenth Doctor and her companions were visiting Mary Godwin, Percy Shelley, Lord Byron and Claire Clairmont in Villa Diodati, while the Lone Cyberman had invaded the house in search of the Cyberium. Though a long-standing enemy of the Cyberman, this was the Doctor's first encounter with the Cyberium; indeed she initially displayed an unfamiliarity with the term.

When the Doctor was finally able to locate Shelley, she had him tell her what happened. She noticed that the Cyberium was burning through his mind and would destroy him if it stayed in him much longer. She referred to this as a "super-computer fused to a cerebral cortex". The Cyberman demanded that the group surrendered the Cyberium, but in order to be extracted, the Cyberium had to leave willingly. The Doctor proceeded to use an old Time Lord trick to show Shelley his future death, which tricked the Cyberium into leaving his body. It floated in mid-air, before flowing into the Doctor. The Doctor stated that the Cyberium had chosen her over the Cyberman, referring to it as Time Lord magnetism and remarked that it looked like she was its true guardian. After the Cyberman threatened to destroy Earth, the Doctor didn't want to take the risk and released the Cyberium to the Cyberman who took it back to the future, (TV: The Haunting of Villa Diodati) in the immediate aftermath of the Cyber-Wars.

Ashad, resurrector of the Cyber-Empire
As the Doctor reasoned, she was recognised by the Cyberium's memory as an enemy of the Cybermen. Believing the Cyberium to see all, the Lone Cybermen felt that it had given him understanding and distilled his purpose, leading him to regard himself as the perfect vessel for the ascension of the Cybermen and the death of their enemies. With the knowledge of the Cyberium, the Cyberman and his Cyberguards found an abandoned Cybercarrier containing hundreds of thousands of dormant warriors. (TV: Ascension of the Cybermen)

The Master's alliance with the Cyberium
was later interested to learn that Ashad was the host of the Cyberium which had created the death particle for him. After Ashad stated that the Cyberium would never leave him while Ashad was still alive, the Master killed the Lone Cyberman with his Tissue Compression Eliminator, releasing the Cyberium. The Master proposed an alliance between them, pointing out that it had all the knowledge of the Cybermen while he had all of the knowledge of the Time Lords taken from the Matrix. The Cyberium agreed and took the Master as his host which he later revealed to the Doctor. However, Ko Sharmus, who felt responsible as he was part of the force that sent the Cyberium back through time and space, sacrificed himself to stop the Master and his new army by detonating the death particle, presumably killing the Master and his army. It is unknown what became of the Cyberium after this. (TV: The Timeless Children)