Triad.

Warning: Sister incest and futa content. If you don't like, please don't read.

In this universe humanity is enslaved to a race of hermaphroditic alien women. Magic and science coexist on the planet, as well as other energies. Physical or yang energy is used and channeled by any living creature, while yoki or monstrous energy is only needed by monsters or yokai.

Yokai come in two groups. Animorphs and undeads. Animorphs are humanoids with animal features. Ears, tails, wings... They also have more active instincts. They are the modern pets, both because of their intelligence and because the old pets no longer exist, since nature and terrestrial ecosystems have been restored to their state before humanity. Undeads are... well, reanimated dead creatures. Skeletons, ghosts, zombies, ghouls... they are the perfect servants, since despite being sentient creatures, they barely have a will since they lack souls. In this world, even human corpses have utility, so funerals do not exist in this imperialist society. Human bodies are taken to specialized enclosures where yoki is used to reanimate them. The resulting undead is random and will lack any memory of their previous human life even though their personality will match what they once were. Any trace of rebellion will be lost, however. Yokai are subservient by nature. For obvious reasons, undeads lack yang. Instead, magic or mana is in charge of keeping them revived.

Demons are made up of three ethereal energies. magic, demonic energy and yin or spiritual energy. they are magically very powerful creatures, but physically fragile. instead of a soul they have an essence. Or several, in rare cases of extremely strong demons.

Magical creatures need magic to exist as they lack yoki and therefore their powerful abilities. It is these creatures that constitute the demons. Their fragile existences allow their souls to be easily infected by demonic energy. If their souls become essences, they will lose their yang and become full demons.

All other living creatures only use yin and yang. There are yoki and demonic energy in their bodies, but they don't use them. Only humans use all four energies, and only to a certain extent. Human females better manipulate mana while males are more attuned to yoki.

The futa, as the aliens have called themselves, theorize that this affinity is due to the unique polarity of human energies. Male humans have negative yang and positive ying, while females are opposite. The negative polarity apparently attracts the other energies better, so the negative yang has a better affinity with the yoki since they are both physical energies, while the negative ying has a better affinity with mana and demonic energy since these are ethereal non-physical energies.

Energy polarity seems to have a direct influence on biological sex as well. The futa, themselves, are of complete positive polarity. Their bodies are fully female even though they possess both fully functional sets of genitalia. They believe that it is this positive polarity that prevents them from reproducing with each other, while they can perfectly reproduce with humans of either sex. This has led them to plan the conquest of humanity at any cost...