From Dynastea Scans:

In the 30th year of reign of King Taesun of the Gye dynasty, when the sickly prince had lost his soul and was close to dying, the emperor ordered the High Priestess Oh Yeon to perform Chohonsul (a ritual to bring the soul just before death back into the body). Because of this, the prince opened his eyes again but he doesn't even recognize his father and starts talking nonsense.

It turns out that the soul that Oh Yeon summoned was none other than Bongmuye, the prince of the first dynasty!

In the end, Bongmuye and Oh Yeon make a deal in which Bongmuye keeps pretending to be crazy and Oh Yeon keeps him as her husband for food while hiding his true identity.

A well-matured soul after a thousand years— Bongmuye, what a great supplement! Oh Yeon is happy and thrilled every day.

Original Webtoon