Incubus means a mythological demon in male form who has s** with sleeping women. One can speculate how women might claim to have been taken by an unknown being. Nevertheless, it is fodder for a Sexual Fantasy.
My late wife and I were into Polyamory and met many men on adult dating sites. MFM threesomes was our favorite, but she willingly let me experiment with her. Her only requirement was, men I recruited had to be older than her son (40ish). But I had a fetish to see a much younger man have six with her.
So I proposed an Incubus Fantasy. My wife would be blindfolded in a dim room and pretend to sleep while an unknown male came in and had complete, bare-back six with her. (She was past menopause and preferred bare-skin penetration.) Her rule for this bed play was, the man had to be someone she "knew". She probably meant "knew" in the Biblical sense, but wasn't specific. We knew, casually, a young (22ish), neighbor man of Jamaican descent who did yard work to pay for college.
My wife had flirted with him and flashed her braless mammaries for my entertainment, but refused to engage with him physically. I don't approve of teasing and felt I owed him more. So I secretly arranged for him to be our Incubus.
The event went as planned. My wife waited, blindfolded while our young Jamaican gardener went in. He snuggled against her naked, caressed, suckled and fingered her until she was good and wet. Then he mounted her and had full, bare-skin penetration while I watched. She willingly spread her legs to take him deeply. He shot his load too soon, but at my urging, continued stroking until she climaxed.
When he finished, he pulled out, dressed and left. My wife never knew who her Incubus was, but said she enjoyed the experience.