Onigamiden - Legend of the Millennium Dragon (2011) on Netflix

Plot summary

In Japan's Heian era, twelve hundred years before Jun Tendo's own time, samurai battle the Oni, figures that appear to be made of smoke, and the monk Gen'un destroys them with his magic powers. Jun, a shy teenage boy, is transported into that past and followed and attacked by an Oni. He hides in Gen'un's temple, where Gen'un tells him that dark monsters are targeting him and explains the war between humans and Oni. Jun wakes at the temple and meets Raiko Minamoto, a skilled swordsman who fights alongside Gen'un. The temple's people welcome Jun as their savior and honor him like royalty, which bewilders him. Jun's left shoulder carries a birthmark, the mark of the Magatama clan, and only a direct bloodline that bears it can awaken the full power of the dragon Orochi.