Let us especially note too that in the practice of this abominable evil, four points in particular are required. First, to renounce the Catholic Faith, secondly, to devote themselves to all evil; thirdly, to offer up children to Satan; fourthly, to indulge in every kind of carnal lust with INCUBI and SUCCUBI and all manner of filthy delights.
Now there are two circumstances which are certainly very common at the present day, that is to say, the connexion of witches with familiars, INCUBI and SUCCUBI, and the horrible sacrifices of small children. Therefore we shall particularly deal with these matters.
There are three chief points to discuss. First, whether the abominable heresies can be multiplied throughout the world by those who give themselves to INCUBI and SUCCUBI. Secondly, whether their actions have not a certain extraordinary power when performed under the influence of certain stars. Thirdly, whether this abominable heresy is not widely spread by those who profanely sacrifice children to SATAN. Moreover, we must consider the influence of the stars, and what power they have in acts of WITCHCRAFT.
With regard to the first question there are three difficulties which need elucidation. The first is a general consideration of these demons, which are called INCUBI. The second question is more particular, for we must inquire, How can these INCUBI perform the human act of copulation? The third question is also a special one, How do witches bind themselves to and copulate with these devils?
–– Heinrich Kramer and James Sprenger, Malleus Maleficarum (a.k.a. "Der Hexenhammer" or "The Hammer of Witches," translated by the Reverend Montague Summers, excerpt edited and abridged by Bard).
The complete text can be found at this link.