basically there is a war going on between the monarchs and the fragment of brilliant lights , the monarchs already invaded and destroyed the earth once but the brilliant lights have a time reversing treasure , so they reversed time and started releasing magic into earth's atmosphere, and the armies of the monarchs thru dungeons and to make humans stronger to resist the war , the war is inevitable in any case so by making hunters atleast a few humans can still survive the aftermath which is why they doin this
the creator(what you can call god) had made brilliant lights and monarchs and made them fight just for his own entertainment, when the lights notice this they rebel and kill the creator , shadow monarch was previously a light but he was loyal and didn't support killing god , after god died he was consumed by darkness and his desire to be able to resurrect thus making him undead king , the war still continued and shadow sided with monarchs to kill his former comrades , but as shadow was a former light the monarchs don't accept him and try to kill him at a decisive stage , thus making him go dormant which is why system is designed to find new vessel for him while both sides are recuperating for the next battle