Uhm, there's actually a pretty long backstory there which will be revealed in the Japan arc, but to cut it short, here's what happened:
What Jinwoo sees is the memory of the Shadow Monarch. The memory of the war between the rulers and the betrayal of demon king Baran is being replayed for Jinwoo to see.
From the start, when Jinwoo woke up from that double lair on the first chapters, his body is being trained by the system so that he can be the "vessel" for the revival of the real Shadow Monarch. Human body actually breaks or decays when a ruler or a monarch try to inhabit them, so the system made sure Jinwoo's body would be strong enough to handle a sovereign's power (hence the quest).
However, what the system did not anticipate was for the shadow monarch to like Jinwoo. He liked him so much that he decided to give all his power and make Jinwoo the next shadow monarch. Instead of "borrowing" the monarch's power, Jinwoo would inherit all of the Shadow Monarch's power. I hope that's clear? That's the simplest way I can explain.
I so freaking confused right now. Can someone explain what just happened in laymen term.cause I do get are they saying the mc is that arise guy from the mind or knowledge or what ever?