3.988 out of 5 from 572 votes
Both of these series are about a guy becoming bound to a very powerful demon/spirit and gaining great power in the process. While there isn't much plot similarity beyond this fact, both series "feel" very similar to one another.
In both Immortal Regis and Kaibutsu Oujo the male lead dies in the very first chapter. Also in both of those series, the male lead gets resurrected as an undead, immortal being by a princess of monster\demon world (his death was partially her fault anyway). His immortality isn't perfect though, if he doesn't regularly receive a dose of blood from the person who resurrected him he will die for good. If you liked that part of plot in one of the series, then you could give the other one a try. Both series also feature various monsters like werewolfs etc. However those series give of a different feel. Kaibutsu Oujo has a fair amount of fun and comedy going on, on the other hand Immortal Regis is rather serious most of the time.