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 described as "the very far future", (TV: Ascension of the Cybermen) the Cyberium was eventually acquired by a resistance force known as the Alliance, who sent it back in time (TV: Fugitive of the Judoon) to Earth in 1816. From here, the Cyberium frequently switched hosts: Percy Shelley was its initial "guardian", before being briefly possessed by the Thirteenth Doctor, who was forced to relinquish the Cyberium to Ashad, the Lone Cyberman. He then time travelled (TV: The Haunting of Villa Diodati) to the immediate aftermath of the Cyber-Wars and attempted to restore the Cyber-Empire, (TV: Ascension of the Cybermen) until murdered Ashad and used the Cyberium to create the CyberMasters. (TV: The Timeless Children)

The Cyber-Wars and the Alliance
During the Cyber-Wars of an era that the Thirteenth Doctor described as "the very far future", (TV: Ascension of the Cybermen) the Cybermen used the Cyberium; 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 late in the Cyber-Wars or during the immediate aftermath of the conflicts, (TV: Ascension of the Cybermen) a resistance group known as the Alliance captured the Cyberium and sent it back through time and space in order to prevent the Lone Cyberman from resurrecting 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 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"
The Cyberium arrived on Earth in 1816. Percy Shelley found the Cyberium in Lake Geneva; when he recovered it, the Cyberium melted into 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 utilised a perception filter on the Villa to keep itself from being discovered. Meanwhile, Shelley acted as its guardian, although the sheer amount of data processing through Shelley's mind, ranging from vast quantities of code to the battles of the Cyber-Wars, began to drive him mad, and so he hid in the Villa's cellar.

Meanwhile, the Thirteenth Doctor, Ryan, Yaz and Graham were visiting Mary Godwin, Lord Byron, Claire Clairmont and Dr John Polidori in Villa Diodati, when the Lone Cyberman - whose original name was Ashad - invaded the Villa in search of the Cyberium. Though a long-standing enemy of the Cybermen, this was the Doctor's first encounter with the Cyberium, initially displaying an unfamiliarity with the term.

When the Doctor was finally able to locate Shelley, she had him tell her what happened, noticing that the Cyberium was burning through Shelley's mind and would destroy him if it stayed in him much longer. Despite his efforts to stop Ashad from teleporting into the cellar, Shelley relented, as per the Doctor's advice, and Ashad confronted Shelley, demanding that the group surrender the Cyberium or he would kill Mary, Shelley's partner. However, in order to be extracted, the Cyberium had to leave Shelley willingly.

The Doctor proceeded to use an "old Time Lord trick" to manipulate Shelley's mind to show the poet his future death, which tricked the Cyberium into leaving his body, believing Shelley to be dead. After disabling the perception filter on the Villa, the Cyberium entered the Doctor's body; the Doctor claimed that her "Time Lord magnetism" implied that she was the Cyberium's true guardian. However, when Ashad threatened to destroy the Earth with his cyber-ship, the Doctor released the Cyberium to Ashad, who instantly teleported to his ship and travelled 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, Ashad 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, Ashad and his Cyberguards travelled to a refugee planet and attempted to kill what few humans were left on the planet, including the Doctor, Ryan, Yaz and Graham, who had followed Ashad. When Graham and Yaz escaped with refugees Ravio, Hedlarni and Bescot in one of the Cybermen's Cyberfighters, Ashad and his Cyberguards followed in a spare Cyberfighter. The trio of Cybermen followed the humans to the ruins of one of the final battles of the Cyber-Wars, in particular to an abandoned Cybercarrier - which the Cyberium had informed Ashad of - and Ashad began reviving the ship's army of Cyber-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)