A Hypothetical History of the Cyberium
(Brainstorming based on stuff Iāve discussed with @lungbarrowmas-autisticus. The idea of the Cyberium originating from the Yssgaroth was totally his idea. The rest is my own speculation).
During the Vampire Wars, the Yssgaroth (Also known as the Great Vampires; either way, scary Lovecraftian fuckers from another Universe) either create or simply just release a sentient liquid metal creature that would eventually come to be known as the Cyberium into N-Space.
This creature contains the conceptual schematics to instigate widespread cybernetic-conversion amongst a planetās populace just by its mere presence (if you think about it, the Cybermen and the Vampires have many things in common; they both convert people often against their will, they both grant their victims immortality, this all coupled with a desire to spread about their kind as much as possible).
The Cyberium can lure in some of its Hosts by subconsciously spreading its influence throughout their time stream upon bonding with them (think The Master/Missyās weird fascination with creating Cybermen throughout the Moffat era), allowing them to create Cybermen with ease.
A partially-converted Cyberman from the Cyber-Wars of the future known as Ashad travels back in time to the Vampire Wars having heard stories of the Cyberium.
As Ashad brings the Cyberium back to the future through the Time Vortex, it spreads its conceptual schematics throughout time and space (particularly amongst many Human-like populations due to Ashadās base genetics). The Cybermen become a universal fact (perhaps even multiversal due to the presence of the Peteās World Cybermen).
Ashad uses the Cyberium to unite the existing Cybermen and bring Humanity to a point of almost total extinction towards the end of the Cyber-Wars.
Surviving members of The Alliance manage to send the Cyberium back in time to 1816 in an effort to stop the relentless advance of the Cybermen.
Percy Shelley accidentally ends up taking the Cyberium into his bloodstream; meanwhile Ashad travels back in time to locate it.
The Doctor manages to liberate the Cyberium from Shelleyās bloodstream, with it favouring her over Ashad due to her superior genetics (and presumably because it was initially designed by the Yssgaroth to convert Time Lord-like organisms). However, after Ashad threatens to destroy the Earth, The Doctor releases the Cyberium from herself, and it rebonds with Ashad.
The Cyberium creates the Death Particle for Ashad, a weapon capble of destroying all organic life on a world. As the Cybermen are part-organic, Ashad hopes that this will allow his forces to āascend {to} full mechanisationā.
Invited to the ruins of Gallifrey by The Master, Ashad brings his fleet of Cybermen through The Boundry. In an effort to release the Death Particle within Ashad, The Master uses his TCE to shrink and kill him, subsequently bonding with the Cyberium himself shortly after. He uses his newly gained powers to create the CyberMasters (soldiers capable of regenerating seemingly indefinitely) from the corpses of deceased Time Lords (side note: The Masterās totally The Hybrid, fight me).
The Master leaves Ashadās miniaturised corpse containing the Death Particle attached to a detonator for The Doctor to use against him and his new army. However, before getting the chance, Ko Sharmus (a veteran from the Cyber-Wars) returns and takes The Doctorās place, using the Death Particle and seemingly killing The Master and his forces for good (however judging by his track record, thatās probably unlikely).
The fate of the Cyberium (as of the date that this post is written) is left unknown due to its artificial nature. However, Iāve got a feeling that if The Master survives the events of TV: āThe Timeless Childrenā, then even if he becomes separated from the Cyberium, he will forever be linked in one way or another to the Cybermen due to the nature of the creatureās way of fusing with its Hosts.