The Seal of the Red Night

HyeonRyeong lives inbetween the world of the living and the world of the dead due to his heart having been switched with Gwi-Ho’s. When HyeonRyeong's village is hit with a string of mysterious murders, a powerful and mysterious shaman-girl named GabJin shows up. And as HyeonRyeong's demonization speeds up, he and GabJin set off on a journey to find Gwi-Ho in order to turn him back into a human.
Source: Hour of Lunacy