Spoiler
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Jinwoo isn't the original Shadow Monarch but rather the power was passed down to him and he became the new shadow monarch(they became one, something like that). The reason why Jinwoo gets his powers in a game-like way is because the shadow monarch and the angel statue being (he's the Architect of the game btw) worked together. Because the powers of the shadow monarch was so great, they made it as though in a game. The shadow monarch betrayed the Architect by being one with Jinwoo. After waking up, Jinwoo defeated the Architect and became more op.
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Major spoiler
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The shadow monarch was once a ruler or you can call it a wingding. But he was not just an ordinary ruler, he was like the head of the rulers or archangel something like that. The God they serve found entertainment in the war of the rulers and demons which kept on repeating and repeating. Later on, the rulers, tired of the never-ending war, realized this and decided to rebel. The shadow monarch didn't participate in the rebellion tho. He served the God to the end. The God was defeated by his own followers. The God anticipated this rebellion told the Shadow Monarch that he gave him a power that will let him use the dead.
So anybody want a spoiler?