A good message, badly executed. The premise is MC is kidnapped into a village of Youkai (monsters) to .ake babies with them, him being the only one who can. It pits MC and the harem of girls who eventually come to love him against the shadowy institution that is the village mothers and their uncompromising demands on their grandchildren. The grandchildren win out, accepting MC being monogamous, even if they won't have "pure" youkai children.
I initially wondered why this story existed in the first place. The MC is given a near-godly opportunity to have his own monster harem, this being the first time any girls took an interest in him, but he adamantly refuses. This seemed really silly at first, but sometime around chapter 30 it clicked. The theme deals with duty versus will, and shows the fundamental flaw of duty. Namely, duty without belief is meaningless, and the painful first 20 chapters or so lead to better chapters near the end which show this weakness of duty.
Despite having a strong message, the story felt painful. Harem is for me, in the first place, a way to read out wish fufillment, to have the power of multiple lovers in my reach. This harem took that power away by making the MC powerless, at the whim of forces outside his control. Thinking about most harem, this is the situation any one girl of the harem finds themselves in. Why so many girls love one unremarkable guy almost feels like they're being forced into the relation by a higher power, in this case the mangaka. As such, Oni ga Deru provides a critique of harem as a whole by making the male MC similar to a female side characters of other harem stories.
Otherwise, the story felt like it really lacked direction for most of the plot. The ending was very anticlimactic and a disappointment in general. While I liked the story's message and theme, the MC was a total pain throughout the entire story. And I honestly don't care about the message. I read harem as escapism, not to be lectured on morals and ethics. As such, this manga was a waste of my time.
3/10.