Quoted:
The succubi and the incubi were female and male (respectively) demons who were believed to prey on mortal men and women (respectively) while they slept.
Can't help you with the flick though. Haven't seen it.
According to the Malleus Maleficarum, A succubi and incubi were the SAME demon. They shape changed after sex as a woman and redeposited the semen as a man. A convenient explanation to why Father Mathus looks a lot like Sister Mary Patrick's son. DEMONS DID IT!!