As the first magicians they pioneered the path for all to come. They brought energy into each other to fuel their miracles. 200 years later that energy is used to fuel the System. The System hungered and the mages could not keep up. All but one. He who decoded the secrets of the system and replaced it with his fleshly body had become the saviour and the overlord of the world. All who wished to hold real power would have to pass his numerous tests. As he was experimenting a crack in reality appeared and horrifying monsters and aberrations would claw their way out. Day by day the crack spread. Inch by inch the crack grew.
Those who survived were the people who received the grand mage's blessings. As previously forbidden magics were researched flesh puppets were born as a broken man sought his vengeance. His puppets were stronger in every way. They were even made completely subservient due to the numerous slave sigils littering the beast's body. Others learnt that after the portal opened their mortal bodies could improve without end as long as someone was willing to break every bone in their body once a day. Those who learnt magic and practiced the arts of improving their constitution were known as battle mages. They were the few who could kill the beasts without resorting to 'evil' magic. There were limits to these improvements though alongside diminishing returns.
A new System was needed and that system would empower all who couldn't kill the aberrations. If this system was not optimised for non mages they would explode as the energy raged throughout their body. Little did they know there was an aberration that looked like them, it talked like them, it acted like them. These kinds of aberrations of bodily flesh had appeared before but they died out as the wizards who were left became better at identifying them. This one though had never appeared before and it drew its power from the same source as the wizards. It had a similar signature to them so everyone believed they were on the same side.
They weren't. As the being grew in power it swallowed all of the energy that maintained the previous system. It used that energy to create the new system from its old world where you could only become more powerful by killing others. This system was more efficient if you killed a being of your own race. This system demolished kingdoms and crushed empires. The world was in chaos until a determined imp wished to rule. He crawled out of the underworld to create an empire that spanned the world.