Threads of Time Vol 2

Threads of Time by Mi Young Noh. A Fantasy manga rated Teen, published in English by Tokyopop.

Moon Gwang-ki is a modern-day Korean teenager who has been haunted by the same dream his whole life. After an accident, he wakes up in 13th-century Korea, in the middle of the Mongol invasion, living the life he's only ever seen in his sleep.