Kuon no Kizuna

This story starts out from the perspective of a high school student. One day a transfer student comes to his school, and tells him she’ll kill him. At the same time, he starts seeing weird dreams and people mysteriously gets killed in his city.
Soon he realizes that them two are incarnated personalities of a man and a goddess that fell in love 1000 years ago. Since then, they have been trapped in a cycle of reincarnation and a battle against an evil god.

[From Akihabara Channel]