The Twelve Kingdoms (Shadow of the Moon, The Sea of Shadow)

Nakajima Youko is your average somewhat timid high school student. One day, a strange man named Keiki appears before her, swearing allegiance. Before she could properly register what was happening, demon-like creatures attack Youko and her friends, after which they are pulled into a different world. A world unlike what she has ever known. Separated from Keiki, Youko and her friends must do whatever they can if they wish to survive in this new world.