An ancient demon was summoned into this realm of existence thousands of human years ago roams free. This demon is millions of years old and yearns to return to its own world. It has found humans boring and distasteful. It has learned that the females has something nice to offer and keep it satisfied.
The demon is best described as half male and female and is capable of successfully mating with humans. It has a odd mating cycle. It only mates once a year. Being driven by instinct to couple with a female, human ones being the best, animals ones are not to pleasurable. The demon is usually only fertile in that mating cycle. It has the ability to since the fertility and menstrual cycle of human females to gauge when it is the best time to impregnate them. When a human female is found at that point the demon will lure and seduce the victim into a sexual encounter. On occasion the human female can resist the seductions and it then becomes a forceful encounter.
The demon can easily overpower any human with ease. It is usually found in its normal form which is about 6 inches high and is invisible to nearly all humans. It will be seen by humans in its second form with is about 6 feet tall. Its usually seen in its slim standard form, but when it is ready to mate or is enraged it swells in size. It grows another foot or so in height and its muscles and girth grow immense. It's penis is only seen at that time growing huge and painfully big. It is hardly a easy experience when you are being fucked by this demon.
When it mates with a human it will always result in a pregnancy due to its selection of suitable mates. Due to the inability of humans to produce proper children for the demons tastes, it rarely hangs around to raise its offspring. It is driven to keep trying each year. The offspring are born with a tale tell defect. They are always born female and with an above average size penis. They are also born with a vagina, but its nearly always so small as not to be usable. So they are unable to bear children of their own and have to live being the sperm donors. This is why they are hard to identify, they are always mistaken for Hermaphrodites.
They children of this demon all have the ability to detect the blood relations of another of their siblings. After several thousand years, the family line goes pretty far back and they are all half sisters. Imagine having a half sister that is 90 years old and you are only 10, mind numbing. They also share in the sexual drive needs. Unlike the demon, which needs to mate once a year. The girls need to have sex on a frequent bases. Nearly in constant state of hornyness, they have to result to masturbation 3 to 8 times a day when entering puberty. This is the time when their dicks grow to the enormous sizes. The growth triggers the sexual stimulation's and thus the need to beat off. The average length for the demons girls ranges between 11 and 15+ inches. Once the girls pass through puberty, the sex drives remains high but not to the point of thinking about nothing else but masturbating. One other trait is the unusually large amount of semen that is produced. When the girls reach the limit to their penis growth the semen produced begins to increase 10x. A pleasurable experience, since the orgasm continues longer in order to ejaculate all the semen. The cocks bet bigger, then they began to have longer orgasms and more jizz... The children of these offspring don't seem to carry the same cock trait, except the boys who are born with above average size cocks too, nor any other signs of a demon father.
It seems a few of the offspring have converged at the same place and same time. Carly Green was totally unaware of her real father. The Franklin twins knowing of their real father along with Kari Cummings. They all discovered one another at the school. Carly finally being convinced of the bond and relationship once everyone dropped their panties. Only Bonnie Franklin was a normal girl. It seems only one child gets the trait, but both suffered the same sexual drives. Like any half demonic siblings they began experimenting sexually with each other. I digress...On with our story....