Immortal Regis [Manga]

One day, suddenly… a night where the moon shines like never before, a boy meets the otherworldly Serin, a denizen of Chaos, only to be killed and then resurrected. Being an Undead, he can no longer exist with humans due to an ancient law of Chaos. Now he must journey to Chaos to learn of a way to return home and save his brother—who, as a side note, is dying of a rare disease.