What I’m curious about is what Hades meant when he said that each time the clock ticks the UW loses a soul. Why would that be, I wonder?
My biggest guess is that the Underworldians are supposed to be able to finish their unfinished business somehow, however Hades probably cursed the world to where none of them would get the opportunity to do so, therefore whoever enters would be trapped forever…least until he grows bored and throws you into Hell. Basically, the Underworld is a game where you cannot win, and his fear of Regina is probably because she’s the most likely unfinished business for the Underworldians that were killed by her, therefore she poses the greatest threat to his kingdom. Well, not just Regina, probably Rumpelstilskin too which would explain why when Belle and Neal resurrected Rumple, Hades wouldn’t have put up much of a fight trading a threat out of his kingdom for the threat’s son.
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor