The planet Mondas at one point, rotated along the same axis of Earth. For those that bothered to know about such things, it was colloquially known as Earth's twin. Some accounts, dating back to time forgotten, suggested it was created by Destiny. The term "Destiny" was broad enough on its own by virtue. The inconclusive reports did not add any credibility to the already tenuous theory. However, any learned mind that found itself meditating on the nature of Mondas would take pause. It was too similar to Earth. The ecosystem, the climate; even the continents, though inverted, were wholly identical. Perhaps it was some grand experiment? Put forth by a force long forgotten? Destiny? Was there some greater game out there, being played at the edge of the collective perception?
The questions raised by musing on Mondas' nature often took precedence over the questions raised by Mondas itself. It was relegated to one of hundreds, perhaps thousands of smaller questions. The questions were all asked, hoping the sum total of all their answers would tell us why we were here; what our purpose in an ever-expanding universe was.
Inane ramblings of galactic philosophers aside, Mondas was significant for another reason: It was the birthplace of the Cybermen. The forming of Earth's moon destabilized the fragile balance that Earth and Mondas shared, and it was knocked off its axis, flying towards the edge of space. The people knew that if they were to survive, they would have to adapt. They would have to evolve. To survive, the Mondasians would have to upgrade. They chipped away at their humanity, shedding more and more of it until nothing remained. And in the end, the Cybermen were born.
One faction opted to stay on Mondas, eventually moving to suck the energy out of Earth and save their doomed world. The others took to the stars, splitting off into dozens of different factions, all with the same purpose: To survive. To save the universe by upgrading it.
The Cybermen became an established galactic presence, crushing entire cities under their cold, metallic boots. They always added to their ranks, like reapers harvesting souls. They grew. Like some swollen mass, they grew. It was only natural that the Daleks came for them during the great war. The Mondasian Cybermen at this point in time had not yet been fought off by The Doctor's first incarnation. They were young, stumbling over themselves.
On that fateful day on Mondas, a day that never should have been, Cybermen would look up into the sky. They cast their eyes to the Heavens, and were met with the gaping maw of Hell. Great saucers slicing through the night-time expanse would begin bombarding them with laser fire. It was an image that would be burned into the collective Cybermen consciousness. Great glowing bolts of blue raining down on them; burning the dead forests, shocking and tearing up the ground, ripping them in twain. It would be an image that across the farthest reaches of space would rally them to this new cause. The Mondasians were confused, at first unable to process what was happening. It was not until their communication units picked up the storm of screaming. One thousand Daleks, all chanting one word:
"EXTERMINATE! EXTERMINATE! EXTERMINATE!"
The Daleks had declared war. The Cybermen would not back down. The Cybermen would meet them with all they had.