
Question for spoiler
Okay, who is the old man? The king? Jaeshin's father? Just random old man? Thank you.

The old man appears at the very beginning and his story is literally the reason why Jaeshin got punished by his dad who turned him into a puppy in the mortal world lol The old man was gambling with Jaeshin and asked him to extend his life. Jaeshin played a game on him by taking his son's life to extend the old man's. Except he wasn't allowed to and his father punished him by sending him as puppy to the mortal world so he can learn some manners and grow a heart! In the meantime, the old man was plotting his revenge over Jaeshin. Of course he didn't take Jaeshin's life when the opportunity came but rather his precious partner just like he took his son! The season ending this way makes total sense. Now we have to wait for Season 4 and see what the author will be cooking for us :))

Like for real, the underworld is doing nothing when things like this run amok?

They just want to see how things play out... originally lee nok was supposed to die but jaeshin changed that so now they also want to see how and what will happen because lee noks future changes with every decision him and jaeshin makeso rather than interfering they just gonna wait and watch...
We should also remember that at first jaeshin had no issues with taking nok to underworld but then he decided not to coz of love and fear that he would forget him like his mom...and that problem still stands....it's a waiting game as to how he will die

Ya the thing is the reason why crown prince dies(lee nok) is due to the empress....how is the dark spirit part...so noks future is directly tied to the existence and well non existence of the said spirit hence it's difficult for the underworld too to interfere in this...
Like he is the crown prince his future affects not just him but the entire nations so no reckless moves can be made so as to anger the wills of heavens
The plane plot is refreshing, the side quest is just getting wilder