The series follows the life of Kai Houjou, the son of a young human woman and the king of demons. Kai's teenage mother was raped and Kai was stolen from her at birth to be raised as a tortured prisoner in the demon world. He has now escaped and returned to the human world to find and protect his mother. However, time moves differently in the two worlds and although Kai is grown, it has only been a year since his mother gave birth. Kai enters his mother's high school in order to stay near her. Meanwhile, demons hunt for Kai, and he must fight or make deals to save himself and his mother, while trying to discover why the demons consider him such a threat.
The story is dark, with references to and scenes of rape, torture, incest, and murder. Kai is a teenage boy who has known little in the world besides torture and rape, mostly at the hands of his older half-brother, a demon prince. Kai's mother is a traumatized victim, who has found the strength to go back to school after her ordeal, even though she is now the subject of rumor and ridicule. Many of the demons have tortured/incestuous pasts (or presents) themselves. While the art is not very sexually explicit, and less violently explicit than one might expect, the story implies all manner of horror, perversion, and eroticism.
As you might guess from the low MAL rating, "Lumen Lunae" isn't all that great. I can't quite decide if it's tasteless, or just cheesy. The plot is extremely thin, the pacing is terribly rushed, and the biggest emphasis is on shounen-ai fanservice.
The story takes place between earth and the demon world (where humans live as food and slave labor for the demons). Kai is a half demon of sorts who is ordered to kill his human mother. If he kills her, everything will be fine for him. When Kai decides he can't kill her after all, lots of demons show up to take care of the job for him.
That's pretty much the extent of the plot. Other things happen in the story, but they are only setups to provide scenes of sadistic torture, or semi-BL...let's say "interaction." All the male characters have a hobby of randomly kissing each other (if not more). Everything is very rushed, and we keep tumbling at a mad rush from one scene to the next. There are also several leaps of the imagination, and lots of improbable things happen (such as Kai's mom guessed he was her son before he told her...but why would she think a high-schooler was her son, when he should have still been a baby?).
The art is very old-fashioned. I'll give it some credit since it's pretty easy to tell the different characters apart. But they are all very sharp-faced and have eyes that are either slits, or too big.
Overall, it's a rather trashy series made for readers who are just desperate for some shounen-ai, and are fine with a story that has practically no plot.