I really wanna know what the hell happened to the first shadow monarch like was he killed ...

SPOILERS AHEAD !! (i may have spoiled more than intended)
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the shadow monarch whose name is 'Ashborn' was known for being the "Greatest Fragment of Brilliant Light". there were 8 rulers and Ashborn was one of the 8 rulers, he was actually the strongest ruler. problems arise between the rulers and the "absolute being" (which I assume is someone of high status) because the rulers found out that their purpose was all due to the absolute being's amusement. 7 out of the 8 rulers went against the absolute being and tried to dethrone the absolute being while Ashborn stayed loyal and protected absolute being 'til his death (he was overpowered). Ashborn did indeed die but the absolute being placed a failsafe in him since he was loyal to the end. Ashborn also has the power of death. fast forward, Ashborn joins the monarchs and more problems arise with the monarchs and the rulers. fast forward again, Ashborn wanted to find someone who could handle his powers, one being the power of death. this is where sung jin-woo comes in (so sung jin-woo isn't actually his descendant or anything). he witnessed sung jin-woo, an e-rank hunter, escape death many times and chose to possess him. the whole purpose of this was not only for him to obtain a human body to pass his powers onto but to also obtain the human body for himself. when sung jin-woo got stronger and stronger, Ashborn was supposed to fully possess his body but Ashborn chose not to, betraying the system. this is how Ashborn and sung jin-woo became one with all of Ashborn's powers transferred to sung jin-woo and sung jin-woo becoming the next shadow monarch.

Thank you for the detailed explantion i love you so much

you're welcome !

Ohhhh my god thank yoouuuu so much for the explanationnnnnn

anytime!