This is the sequel to Immortal Regis, so you shouldn't read this before reading that. Confusingly, this series starts off many years after the original, with Jae a grown man; not only that but it starts out following some bratty new undead, and it's not clear who's who for a while; nonetheless once familiar faces start showing up, the series really takes off. Point of note: the school is never mentioned again.
The tone of this one is very, very dark. Perhaps no plot point illustrates this as well as what Serin is now suffering: a fate "worse then death", bound to a cross and subjected to an eternal nightmare under what appears to be immense sedation at the hands of a grownup Jae, who has taken to calling himself Nex, for a mysterious betrayal that few know about and none want to explain to us incensed readers (there was a war, she stabbed him in the back somehow, that's all we know). The new Jae is a frighteningly cruel high king; a wholesale slaughterer of all who get in his way, with an extremely callous attitude towards others and no trust for anyone but his stone-girl, but of course the hardest pill to swallow is what he did to the girl he once loved so deeply and instinctively: after all, one girl is a tragedy, but one village just a statistic. As the story progresses familiar faces appear across divided lines, for and against Nex (but mostly for), as well as one new puppetmaster who seems to be into causing trouble just for the fun of it.
Midway through the series we are treated to an extended flashback (two whole volumes) answering a lot of questions about what the heck is going on, showcasing events from two years after the end of Regis, but frustratingly ending before answering the most important questions, why is Serin sealed away in hell, and why is Nex acting like such a perpetual bastard.
Overall, this is a distinct departure from the prequel, and a very interesting, "far future" story structure. Given that the prequel ended in a plot hook, this is a must-read for anyone that read the original. If you liked the original...you'll probably (possibly resentfully) like this one too.