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.