I was slightly surprised to not find this on the master list of series. Thus, I felt the need to inform FAKKU! about this great series.
BACKGROUND INFO: JUDER is the artist, and Ga Un-Bi is the writer. This is a manhwa, meaning that should you search RAWs, it'll be in Korean, as it is Korean-made. The entire series is announced as a trilogy, with three parts of different names. Immortal Regis is the first part of the trilogy, with Cavalier of the Abyss being the second part. The third part is yet unannounced, as Cavalier of the Abyss is not finished yet.
Immortal Regis:
The first part of the trilogy. Introduces the main characters. Consists of 6 volumes spanning 44 chapters, all translated.
"One day, suddenly... a night where the moon shines like never before, a boy meets the otherworldly Serin, a denizen of Chaos, only to die and become the undead. Now immortal, he can no longer exist with humans due to an ancient law... A tale of revelations that begins as he returns to Chaos, the start of a new adventure! "
-OM summary.
Read at OneManga!
Cavalier of the Abyss:
The second part of the trilogy. Readers will likely be confused of the events between Immortal Regis and Cavalier of the Abyss until they've 1. Read the RAWs(like I have) or 2. Wait for translations. I can tell you that they begin clearing things up around volume 2. Currently 3 volumes have been released in Korea, and 3 chapters have been translated.
No summary, as I can't really summarize anything without spoiling things.
Read at Onemanga!
Enjoy.