What the Hell are Incubi and Succubi?
They are incorporeal sexual energy feeders, generally regarded to be a type of incorporeal vampire.
In a masculine form, they are called incubi (singular: incubus), and in a feminine form, succubi (singular: succubus).
What do they look like?
Incubi and succubi have no specific appearance, generally taking their form from the mind of the person they're feeding. They rarely materialise physically (although they can be induced to do so through various rituals), preferring to appear in the dreams, or (more rarely) to induce sleep paralysis in the victim. They may also be encountered on the astral plane.
Their appearance in dreams and astral states can be hard to discern from more conventional dream images, but it is worth noting that they are not good at the details of human anatomy, as they don't really understand it. Nipples, navels, similar apparently minor cosmetic details will often be blurry or indistinct, even when the entity is in clear focus otherwise. Such details may also be completely missing.
When evoked into the physical plane, they will often not have a very clear appearance. Their evoked energy form will be visible and tangible to one trained in seeing such things, but they might even go unnoticed by the untrained eye.
What do they do?
They drain energy from their victim by causing or taking advantage of sexual arousal or frustration. In the middle ages, monks, nuns, and other celibates were particularly unfond of these beings, as they manifested to them with some frequency, due to the sexual frustration which is often an innate part of celibacy.
Incubus and succubus experiences are usually pleasurable for the victim, and will only cause any problem if they are already extremely ill or weak, although they may not. Some vampires and vampiric magicians dislike these beings as they hate being drained of vital energy.
It seems to only be rarely in modern times that an incubus/succubus experience will not be regarded as pleasurable. Generally, these will be situations in which the entity explores the hidden sexuality of it's victim (for example, appearing as a male to a homophobe). This exploration of hidden and forbidden sexuality is part of what made the Christian church revile them to such a great extent and regard having sex with them as so sinful.
Thomas Aquinas, darling of the Catholic Church and 13th Century founder of "scholasticism," believed that devils, in the form of succubi, performed fellatio on sleeping mortals. Then, as incubi, they used the seed so obtained to impregnate virtuous unmarried women in their sleep, thus producing half human/half demonic monsters. In fact, this was a convenient excuse for unmarried women who became pregnant, more often than not. Incubi and succubi are evidently incapable of fertilizing humans in any normal sense. However, this belief does inspire certain possibilities in relation to creating a magickal child of some description.
How do they come into being?
There are a number of theories as to how these beings are created in the first place. Some schools of though theorise that they are created by the more powerful beings they are often found serving, such as Lilith. Magicians of more materialistic paradigms regard succubi to be entities created by their victim's own mind.
It has also been theorised (possibly after the consumption of large amounts of intoxicants) that a magician could cooperate with a succubus to create a Tulpa or Moonchild, which would be a mating of sorts. Such a thing could have a number of results, and the method can be adapted to create both corporeal and incorporeal beings with innate succubitic tendencies.
How can you get rid of them?
They can be prevented from attacking by using substances such as salt and gold, and may be got rid of with a simple banishing such as the Lesser Banishing Ritual of the Pentagram or one of it's derivatives (the Gnostic Pentagram, for instance). Dream catchers are also generally reported to be effective against their attacks upon the sleeping. They can be eliminated by being completely drained of energy, although this is blatant cruelty towards a largely harmless entity.
Where do they come from?
Regions of origin: The terms "incubus" and "succubus" are European in origin, but similar beings appear in the mythology of most regions of the world. As they are incorporeal, geography is largely irrelevant. They are related to (although possibly not the same as) beings such as Mara and Night Hags, which, while they also take their appearance from the minds of their victims, are often more nightmarish in their appearance, feeding on fear rather than lust.
Are they dangerous?
No, not really.
On rare occasions, swarms of these entities will attach themselves to one person, and will be able to physically affect them. As ever, this experience is generally harmless and pleasurable assuming the victim is healthy and suffers no strange repressions regarding sex. It can be rather exhausting to them, though. Under such circumstances, banishing and the use of appropriate prophylactic talismans is recommended should they subject wish to get rid of the beings.
Some mortal vampires and rather more vampiric magicians can project themselves into dreams in order to feed in a manner similar to that of succubi. It is also possible for corporeal vampires (of various sorts) and vampiric magicians to feed on sexual energy in just such a manner as the incorporeal succubi and incubi. Such magicians can also evoke, invoke, and summon succubi, among other entities, and can "create" one with a specific form to send to a chosen victim.
Incubi and succubi often form an entourage to more powerful entities, particularly those entities with vampiric and sexual associations.
These beings are attracted by the use of magick as well as by sexual desire and frustration. Once they decide they like someone, that person will generally receive multiple visitations over the course of his or her life. Some people regard the exchange of a little energy for the sexual pleasures offered by these beings to be well worth it, and will seek to encourage visitations by them.