2012 Catastrophe

The 2012 Catastrophe, (also called Doomsday, Beginning of the New Age or Current Era, End of the Old Age or the Ancient Era or just simply 2012) was a catastrophic event that occurred on December 21, 2012. The event was so traumatic that all human life faced risk of extinction.

Radiation poisoning
Minutes after the impact, the debris from the collision landed & nuclear radiation was slowly covering the Earth. In less than 12 hours, the population of humans on Earth plummeted from it's 7 billion to a mere 100. By the time a year had passed, only 50-something on those humans survived & were able to reproduce to continue the human race.

Alitronium
Alitronium is a radioactive substance that falls between the elements of uranium & neptunium. It's radioactivity is so intense that a single cubic millimeter can spread waves as far as 50 square miles. The substance would later be studyed by nuclear researchers at the Radiation Factory, where there is a silo built over the factory that contains more than enough to kill everything on the planet just by opening the top of the silo all the way. This is because the more there is, the more lethal the radiation is, the stronger the waves are & the farther it can go.

Conditions for life
The life conditions were so horrendous after the explosion that it was similar to the dawn of mankind, in which disease was common & fatal & life was so harsh that only half of the children of the survivors would live to adulthood. Humans, however, bested nature & began to restart society by the time year 2050 rolled around.

Aftermath
The impact left the human civilization on the brink of extinction for several years, reducing the world population of 7 billion to a mere 60 or less. Notable metropolises that didn't sustain tons of damage during the catastrophe became ghost towns that were to be repopulated later or left uninhabited while others that did sustain damage were either destroyed or became large ruins. The years between 2013 & 2050 are considered the Age of Lost History as there were so few humans left on the Earth that history had stopped writing. During this time, the alitronium somehow caused the rise of shadowghosts, evil spiritual beings who manifested themselves as monsters that are half metal, half ghost & can possess humans & hide inside them (as though hiding in the human's "shadow"). These creatures were fought off in a huge war that lasted almost quarter of a century. They were raised again & resulted in a second war that lasted longer.