It would be nice if the author had given us a little bit more of explanation, but oh well. I believe their souls are intertwined and they can't live without each other once they meet. I believe their fate is that they will always choose each other, doesn't matter the consequences, that's why they can't know peace and will be trapped in the underworld as sinful souls. Jo was given the chance to break the pattern after working for the afterlife and was given a life without his past memories, but the king's appearance made him choose him again. So the shaman performed the Gut to summon the king, aware his body would perish in the process, at this point I believe the king is a very strong and dark soul, I believe the shaman knew summoning him would cause his death. When he died and immediately became a ghost, his memories were recovered, as he was no longer human, and he followed the king to the underworld, as they have no desire to reincarnate, just to be together.
It would be nice if the author had given us a little bit more of explanation, but oh well. I believe their souls are intertwined and they can't live without each other once they meet. I believe their fate is that they will always choose each other, doesn't matter the consequences, that's why they can't know peace and will be trapped in the underworld as sinful souls. Jo was given the chance to break the pattern after working for the afterlife and was given a life without his past memories, but the king's appearance made him choose him again. So the shaman performed the Gut to summon the king, aware his body would perish in the process, at this point I believe the king is a very strong and dark soul, I believe the shaman knew summoning him would cause his death. When he died and immediately became a ghost, his memories were recovered, as he was no longer human, and he followed the king to the underworld, as they have no desire to reincarnate, just to be together.
But then who is Ja Cheongsam ( sorry if tge spelling is incorrect) that even in the last episode demon King takes this name.
That was his name in his previous life, when he served the king