Spoiler...
For the gates with specific keys, it is Osborn who is behind it. He’s the previous shadow sovereign and basically powering up our mc so he can take over but he decided not to and fuse as one with him. For the whole gate situation in the world that is caused by The Rulers. Basically the earth is a war ground for the war between the Rulers and the Sovereign (the shadow sovereign is a rebel so our MC stands with the ruler). The reason why The Rulers put gates in the world is to prepare humans not for the war but just to survive haha. The hunters are use to mana and as seen in some cases normal human can get the condition of enteral sleep. The reason why our MC was chosen because he was similar to Osborn in the sense he was trending near death (when he was E-rank) and as a person who was the embodiment of death it was perfect. He meets his dad when he meets up with two of the sovereign and has a fight (same time his true power comes).
Sorry meant to upvote
i forgot who osborn was?
The former shadow monarch, the one replaced by jinwoo.
Or rather, going to be replaced by Jinwoo, since we haven't got there in the manga yet.
who is behind all of this? What do they get out of this and why is it that he is the only special one? Also when is he going to reunite with his dad? ahhh!!! so many unanswered questions ┑(￣Д ￣)┍