The Rebinding

The events that lead up to and after The Severance caused a terrible tear between the fabric of our existence. So much so, that it is a miracle that we are even here in the first place. That miracle is oft referred to as The Rebinding. 

Now whether or not The Rebinding hold water is a topic for a more educated person than I but legend has it that whatever happen, it is the reason why we have half breeds.