the monarchs are creatures of chaos, rulers are creatures of light.. all of them are created by a god to fight each other. the rulers (except for 1) kill the god because they find out that god make all of them (monarchs and rulers) simply to fight each other unendingly for entertainment. the one ruler that didn't kill the god "clipped" his own wings and become the monarch of death. he went to war in the monarchs side to fight the other rulers for revenge, but the rulers don't want to end him, rather they seek his forgiveness (he was one of their best)... the war ended some monarch escaped to chaos realm till they find earth so they want to use earth as their place.
the rulers find out late and tried their best to prevent it but they failed to save humanity. so they use one power of the god that they stole and rewind the time for 10 years and redo it, but failed again.. this was done several time but the end result are the same: despite them winning the war, humanity and the earth still got wasted (because earth is filled with weak lifeforms that knows no magic). so for the last time they rewind the time for 10 years and start opening these gates and letting mana to flow through the surface of earth, blessing some to gain power, (almost like terraforming the earth) so that when the time comes and the huge war ended at the very least these "hunters" still survive and can repopulate the earth.
where as for their relationship: initially these ruler doesn't want to face the monarch of death again (he really is one of the best of both ruler and monarch) so initially they want him out of the upcoming war. the monarch himself is fond of the MC and don't really care about the other monarchs... so he let the MC choose what to do with his power. the ruler eventually finds out about it so no problem there..
so now we have a human that have a monarch power who was originally a ruler facing the upcoming war