Manga by author Ga On-bi

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 is dying of a rare disease.

Sequel to Immortal Regis. Reading the entirety of the prequel is highly recommended, otherwise, this story from chapter 4 on will spoil it and/or be confusing. Cavalier of the Abyss (so far) is set only in the world of Chaos. There is now a Regis of Chaos.