I think it's about them burning from the Qi accumulation caused by their condition.
And how can they cast off?
Yin is cold and yang is hot. They will explode
Yin is cold and yang is hot. They will explode with their qi. Soun will burn to death and Cheonho will freeze to death. But if their energy come together they will be reborn meaning casting off their punishment. (▰˘◡˘▰)
So they have to do the deed right ??
What is that divine punishment and how can they cast off??