Basically yoen bound kihoon soul in their previous life as a child unaware that kihoon was destined to be alone and shine alone, while yoen had fated partner of his own. Bc of what he did kihoon was cursed with misfortune which ruined his life both in previous incarnation and his current. The rock sent yoen back in time to his previous incarnation to learn about what he did and how more or less he can fix it but if he fails to do so him and kihoons memories of each other will be erased completely.
My theory is the rock sent yoens current incarnation to relive his mistake of cursing kihoon and he has to fix it and timeline so kihoon can live a better life with being cursed with misfortune. I think the rock is trying to teach yoen the lesson of if you love kihoon, you’ll let him go so he can live the life he was destined to live or you can be selfish and ruin kihoons life forever.
I’m pretty sure him and kihoon are supposed to be friends in each other lives not lovers but bc of the force thread tie, yoen and kihoon did get what they want which was each other but they also(more like yoen cuz he’s likely gonna die as seen in chapter 16) payed the price for it

Yall i got lost, why did they go back in time