Wikipedia Contributors, “Succubus,” Wikimedia Foundation, last modified December 25, 2025, 21:12, https://en.wikipedia.org/wiki/Succubus.

Wikipedia Contributors, “Incubus,” Wikimedia Foundation, last modified January 28, 2026, 21:31, https://en.wikipedia.org/wiki/Incubus.

The universe is divided between the Earth and the spirit world.

A soul is the ability to physically interact with the particular world that it belongs to. A human’s soul is part of the Earth; a spirit’s soul is part of the spirit world. A human being’s earthbound soul can only directly interact with the Earth; a spirit’s astral soul can only directly interact with the spirit world. Double-souled beings possess both an earthbound and an astral soul, and they can physically interact with both worlds.

A mind is the ability to perceive and communicate with other minds. Some humans and spirits can release their minds from their souls and travel between other conscious beings’ minds, regardless of whether those beings’ souls are earthbound, astral, or both.

A demon transfers energy from the human world to the spirit world by converting earthbound energy (kinetic, chemical, electric, etc.) into astral energy. An angel transfers energy from the spirit world to the human world by converting astral energy to earthbound energy.

Succubi are plants. Their souls are astral, not earthbound, and every succubus begins its life as a bud on a tree in the spirit world. The tree provides the buds with astral energy until they reach maturity and fly away. At the end of their lives, succubi return to the tree to die, and the tree consumes their bodies.

Mature succubi sustain themselves by consuming the energy they drain from human souls. When a succubus hunts, it releases its mind from its soul and floats through the stream of human consciousness, projecting a lure into the minds it passes through. The lure is an abstract concept created by the succubus to attract human prey. For example, a lure could be the image of a beautiful woman, and humans in the succubus’s wake might see her in their peripheral vision, in crowds, in reflections, or their dreams.

The lure will hook the first human who thinks of it intentionally. If someone has a dream about the woman, but they forget about it, the succubus won’t feed on them. However, if they dream about her and then purposefully think about her again the next night, the lure will open a connection between their mind and the succubus’s. Through this connection, the succubus will sense what the human finds attractive and use it to make the lure more appealing.

All of the earthbound energy the human expends in actively thinking about the lure is transmuted into astral energy and consumed by the succubus. The more the human thinks about the lure, the more strongly it will connect their mind to the succubus’s, and the more clearly the succubus will be able to observe the human’s mind. The human prey is trapped in an escalating cycle of obsession as the lure burrows deeper into their mind. In the late stages of the hunt, the human loses the ability to do anything but think about the lure: all of the energy they could have used to break the obsessive loop has already been consumed by the succubus. They die of thirst, hunger, or exposure.

Incubi are mammals. An incubus is double-souled, with an astral and an earthbound soul, and an incubus can be born either to a pair of incubi or to an incubus and a human. Incubi appear mostly identical to humans, but their bodies are as present and tangible in the spirit world as they are on Earth. They sustain their earthbound souls like humans and their astral souls like succubi. A young incubus consumes astral energy by drinking sap from the succubus tree. Mature incubi hunt similarly to succubi, but they drain energy that humans use to think about their earthbound souls, not abstract lures. When an incubus dies, their earthbound body decomposes like a human’s, while their astral body’s remaining energy dissipates across the spirit world.

Devils are demons of species that drain earthbound energy from human beings and who have chosen to feed on humans selectively, according to a code that defines which humans deserve to have their energy drained and by how much. A devil order is a group of devils bound by a single code. Most modern devil orders’ codes prescribe draining energy only from humans who have acted immorally. Most modern incubi are devils; most succubi are not.

There are worlds besides Earth and the spirit world.

Incubus lifespans range from one hundred fifty to two hundred years, and succubus lifespans range from less than two months to over three thousand years.

A full-grown succubus flower ranges from forty to sixty feet in length.

Incubi could shift back and forth between Earth and the spirit world, or the two realities could be superimposed, with the incubi equally present in both at all times. Incubi and other double-souled beings could also act as anchor points between Earth and the spirit world, and the relative positions of the two worlds could shift around them.

Most incubi live in towns that resemble Earth’s cities, villages, and subdivisions, but which are built in the spirit world using its endemic technologies and are home to minority populations of other demon species.

A story set in this world could be a coming-of-age story centered on a group of young incubi growing up in the spirit world. Some of their parents could be incubus couples living in the spirit world, some could be incubi who spend most of their time on Earth, and some could be incubus-human couples. Some of the young incubi could be struggling to choose between devil orders, while others question whether they have the right to enforce any order’s code on humanity. Each member of the group has a unique perspective on humanity and succubi, the two species they share their souls with: the humans they resemble but feed on, and the succubi whose needs they share but who they may find frighteningly alien.

It could be possible for human souls to travel to the spirit world, potentially as an effect of becoming pregnant with a double-souled child. Some humans could live in incubus cities in the spirit world, and some could live in parts of the spirit world isolated from incubus society. Some human characters could lead double lives on Earth and in the spirit world, and some could leave Earth permanently. If becoming pregnant with a double-souled being lets a human soul enter the spirit world, some humans could conceive incubi specifically so they can travel between the worlds, and others could develop the ability with no warning.

Another potential plot hook could center on Character A, a human whose soul begins to bleed into the spirit world after unknowingly becoming pregnant with an incubus. People who look subtly off appear in crowds; they notice buildings they can only see from one side of the street; when they walk past windows, they see people walking behind them, but when they turn around, no one is there. Their visions escalate until they learn they’re pregnant with a double-souled child. As their pregnancy progresses, it becomes easier for them to explore the spirit world and interact with the beings who live there, but they also become more aware of the implications of raising a child who will need to spend much of their childhood in the spirit world and who will grow up to feed on human souls. Any religious objections A might have to aborting the incubus could be played for drama or comedy. (Optional subtitle: Pregnancy Isekai.)