Incubi and succubi (singular incubus and succubus) are creatures with sorcerous power, who use humans to reproduce. Each of these creatures is both an incubus and a succubus; 'incubus' is the word for their male form, and 'succubus' refers to their female form.
They generally reproduce by visiting a sleeping man while in succubus form, and engaging in intercourse with him.
The man's sperm is then changed over a period of several days. At the end of this period they turn into an incubus, and visit a sleeping woman, who they impregnate with the sperm.
If a child results it will appear to be human, but will in fact be an incubus/succubus. It will stay in the form of one gender (and indeed does so until puberty). When the child is a few months old the parent will visit the woman again, and steal the baby.
Unsurprisingly they are hated and feared by humans. Thus the belief that a few drops of their blood, dripped into the mouth of a sleeping person, will cause them to sicken and die, may be a myth.
The Thirsty Rose clan have a slightly different method of reproduction. They seduce humans in their waking hours, and during the act spit a blood-red egg into their victim's mouth. If the victim is a woman, they will become pregnant (regardless of their age and fertility). If the victim is a man, they may pass the egg into a woman during sex, and that woman will become pregnant.
Another clan, the Cavaliers of the Slums, likewise seek out partners during their waking hours. They entice men into having sex (the quickest and easiest part of the process, they claim). Tiny parasites attach to the male's organ. When he next has sex with a human woman, the parasites enter the womb, where they develop into a baby. This clan still takes male form for half of every month, but they do not seek intercourse in this time.
The sacred texts of all clans are the Scrolls of Going Forth By Night. They are said to have been written by the first of their kind, the Red Mongrel.