@Mikasa yukii
don't read the spoiler if you dont want to know what happens to the twins & how special their birth is
Spoiler!
Spoiler text. the black king is not hades or anyone in the tower, the tower is used as a nail in the black kings coffin, the gods were locked up in order to "add more weight" that way the tower acts as a prison that hold the black king down & keep him asleep, the heavenly demon is the one who defeated the black king & created the tower to surpress him, everytime yeon-woo destroys a stage/floor in the tower the darkness he sees is the black kings power that was sealed away, if yeon-woo & jeon-woo were never born the black king would've never awakened in any way but due to the circumtances of the twins birth, he does wake up, jeon-woo is "day", which caused the black king to enter a half awake & half asleep state (rather than just being fully asleep) but when yeon-woo entered the tower the black king gained 2/3rds of his consiousness making yeon-woo "night", it's really messed up but im tired of typing this out so read the novel to find out the details