The last thing you remembered is drowning in a river in your attempt to save a kid, before you had the chance to save the kid, you drowned and black out

Moments later you emerged from the river, and found yourself in somekind of traditional village, you have travel 1000 years back in time and lands in the era of Goryeo Dynasty of Korea

you notice someone on his black horse, staring at you from a far

he was the cruelest prince in the era, Baek Ah.