Cyberium

Cyberium was an AI which contained all the knowledge and future history of all cybermen. (TV: The Haunting of Villa Diodati) The Alliance sent the cyberium and a Lone Cyberman back through time and space in order to prevent the return of the Cyberman 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 send the Thirteenth Doctor a warning, telling her to beware of the Lone Cyberman and not give it what it wanted. (TV: Fugitive of the Judoon)

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.

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 "the cyberium at the heart of it, controlling data, strategy, decision-making" and 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 execute its host. 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. (TV: The Haunting of Villa Diodati)