Chapter Text
It begins on a sunny day in a traditional Japanese house where two people reside; one is a brown-haired, magenta-eyed skinny teenage boy wearing a short-sleeve branded T-shirt and black shorts, and the other is a long black-haired, purple-eyed voluptuous woman with a mole under her left eye in a dark sundress appearing to be in her 20s, with the latter almost finishing cleaning the former's ears.
Woman: Do you feel itchy anywhere?
Boy: I'm fine.
Woman: There, all done. You're all clean now.
The boy introduces the woman's name as Chiyo, his older sister, in the narration.
Chiyo: I'm going to make us some food, so be sure to do your homework, Yuu-kun.
Yuu: I can't play any games~?
Chiyo: After you finish your homework.
The boy, now known as Yuu, then exposits his backstory, in which his distant relatives take him under their custody after losing his parents to an accident. However, said relatives moved into a facility home, leading to Chiyo eventually taking him under her custody. He walks into the kitchen to find Chiyo prepping to cook as she starts chopping an eggplant.
Yuu: Chiyo-nee, thank you for everything you've done for me...
Chiyo: My, what's gotten into you all of a sudden~?
Yuu reminisces about the bonding moments he and his sister experience, such as her walking alongside him after their shopping trip, her giving him lunch, and her assuring him in his sleep.
Yuu: So, I've had enough now. I'll fulfill the contract.
Yuu states that Chiyo is his definition of an ideal older sister, but he knows she isn't his actual older sister.
Chiyo: Are you sure?
Chiyo turns around to reveal her actual self: A Shub-Niggurath, also known as the Lovecraftian Black Goat of the Woods with a Thousand Young, taking on the form of a demonic-looking, yet still luscious woman with medium-sized curved yellow-green horns on her head, slightly pointed ears, blue tentacles appearing on the ends of her black hair, purple eyes with pupils resembling that of a goat, black forearms, black thighs with yellow-green goat hooves for feet and a small fluffy magenta and purple striped tail. She wears what appears to be a white virgin-killing sarashi-like leotard with a magenta and purple striped scarf-like attachment that has demonic fangs growing under and similarly colored scarf-like accessories around her thighs that have devilish bones growing above to give off the impression of her wearing unusually-looking thigh-highs. Such an outfit shows that her buxom bust is just a grab away from exposure, and her crotch is barely covered, displaying a considerable amount of pubic mound for Yuu to see.
Chiyo: Thank goodness. Honestly, I wasn't sure how much longer I could hold back. ♥ Well then, as per the contract, I'll be taking something very precious to you, Yuu-kun.
Yuu becomes scared to be devoured by someone like Chiyo despite assuring himself the great, albeit short, time he spent with her before his inevitable end. His end, however, is what he does not expect.
Chiyo: Mnn. ♥ Mnh. ♥ Nh~. ♥
Yuu is surprised that his eldritch older sister has chosen to kiss him, lick his tongue with her own, and slurp his saliva to the point his mind nearly goes blank about how sweet and warm it feels when they stop to catch their breaths.
Chiyo: Mh. ♥ ♥ Nh~. ♥ Nh. ♥
Yuu: Uah. Chiyo-nee, weren't you going to eat me...?
Chiyo: It's not your life that I want, Yuu-kun.
Chiyo then unzips and pulls down his shorts, revealing his above-average-sized cock to her delight as she stares at Yuu with hearts in her pupils.
Chiyo: What I'm taking is "potential." In other words, it's your sperm that I want, Yuu-kun. ♥
Yuu: MY S-...!?
Chiyo interrupts him, recalling a moment she overheard Yuu beating off with her underwear and mumbling out to her in infatuation while rubbing her crotch through her dress lustfully.
Chiyo: You know, I've actually always known about how you masturbate using my underwear. I'd always think about how unfortunate a waste it was. You could have just given it to me. ♥
Yuu covers his eyes with both hands, mortified and humiliated at being caught in the act without him knowing until now.
Chiyo: Ufufu. But now I don't have to hold back anymore. ♥
Chiyo pulls apart her sarashi-like wrappings off her chest to show off her pillowy gorgeous G-cup-sized tits with decently-sized pink areolas and nipples for Yuu to suckle.
Chiyo: You'll cum lots for me, won't you?
Yuu: Y-Yes!
Chiyo: Fufu. Good boy.
Yuu: UWAH!?
Chiyo head-pats and breastfeeds Yuu with one of her hair tentacles coiling and rubbing around his penile shaft in a nursing handjob position.
Chiyo: Does my hair feel good?
Yuu: Nh. Nh~. It feels too good. I'm gonna cum.
Chiyo: Go right ahead. ♥ Make sure you shoot out lots, okay?
Chiyo soon speeds up her tentacle hair that is currently stroking Yuu's cock while assuring him to do his best to cum.
Yuu: Hyah? Aaaaaaaaa...
Yuu found it slightly jarring to see Chiyo, who had just cleaned his ears while resting on her lap moments ago, now making him ejaculate a lengthy rope of semen with her tentacle while lightly pinching one of his nipples.
Yuu: UAH-AH-HYI-AAAAAAA...!!!
Chiyo takes the semen-stained tentacle and lets said semen fall on her tongue to taste it while her other tentacles coil around her busty breasts.
Chiyo: It's so thick. ♥ It's like jelly. ♥ I want more of your virgin semen...
Yuu: But I just came...
Chiyo: That's fine. ♥ Demon powers are pretty amazing. ♥
Chiyo goes down on her knees and begins to suck Yuu's still erect penis by licking him up from the shaft to the glans while coiling and rolling his testicles with her fingers before gradually shoving it into her mouth.
Yuu: Uah! Ah! Hyah! AAAAAAAH!
Chiyo: Ahm ♥. Nh... ♥ Fuu. ♥ Nh... ♥
Yuu notices Chiyo's tongue has tiny folds that entwine and stick to his cock as she sucks on his member like it is delectable to her and does so further as she lays down stomach-first.
Chiyo: Mn. ♥ Mn. ♥ Nph. ♥ Mnnnn. ♥
Yuu: Aaaaaaaa...O-Onee-chan...I'm gonna...I'm gonna cum in your mouth...! Aaaaaaa...
He soon realizes her tongue is coiling around similarly to her hair tentacles, meaning she is about to absorb everything out of him, not helped by her sucking him so quickly that he is about to jizz quicker than he had from the tentacle job.
Yuu: FUAH!
Yuu's cock spurts into Chiyo's mouth as streams of semen hit the back of her throat and flow down her esophagus. He stares at her in astonishment at the fellatio she has done before losing himself to pleasure for a moment while in a weeping expression.
Chiyo: Mn~! ♥
Chiyo slowly pulls his spurting cock out of her mouth, slurping some of his cum before opening it to show Yuu the remaining amount.
Yuu: Hya! Ah! Uaaah!
Chiyo: Haa. ♥ Haa. ♥ ♥ Amaaazing. ♥ You let out sho mush. Well done, well done. ♥
Chiyo savors the taste of Yuu's semen momentarily before gulping it down.
Yuu: Aah! That's dirty...
Chiyo pats Yuu's head a couple of times.
Chiyo: That's not true. Your semen is rich, fresh, and really delicious. ♥
Chiyo stands up with a couple of her tentacles reaching for her crotch.
Chiyo: I want you to shoot out more of that thick seed of yours... ♥ This time in here. ♥
Yuu sees said hair tentacles slide the crotch section of her leotard, revealing her wet vagina as she splits her labia apart to see a slightly gushing opening and a lightly tensed clitoris.
Chiyo: I'm a demon, so I may be slightly different from humans down here, but...
Chiyo momentarily undertakes a menacing mood, her face darkening and her eyes taking an uncanny white glow.
Chiyo: I'm sure it'll feel so good that it'll make your soul flow right out of you. ♥
She lowers down in a cowgirl position with her cunt slightly touching Yuu's penile glans while staring at him with a passionate look.
Chiyo: Okay? You'll grant my wish, right? Use me to graduate from your virginity. ♥
Yuu cannot utterly deny Chiyo's offer as her vaginal lips slowly engulf his penile glans, and his nerves skyrocket for the impending coitus.
Yuu: Uah-ah. It's going in. Ugh-ah-ah. Hyi--AAAAAAAAH!
The initial penetration, combined with the sensitivity of his two previous ejaculations, makes Yuu feel extremely aroused with a starry-eyed expression to the point that he splurts a large amount of semen into her uterus and gushes some out of her pussy.
Chiyo: Ah! ♥ You came already? It's twitching so much.
Yuu: I-I'm so sorry...
Chiyo: It's alright. ♥ You are just too cute, Yuu-kun. ♥
Chiyo rubs a nervous Yuu's head while he grabs and suckles her huge boobs to further her reassurance toward him.
Chiyo: Onee-chan is going to cast a spell on you...To make sure you can slap yourself against me and feel really good, okay? ♥
Chiyo's tentacles coil around and squish Yuu's testicles to prevent any further premature ejaculations.
Chiyo: The pleasure and amount you cum will be multiplied indefinitely, though. The spell I'm putting on you won't let you shoot your stuff out until I give you the signal.
Chiyo subsequently tightens her pussy to further the spell, leading to Yuu bucking his hips out of sexual shock.
Chiyo: There!
Yuu: HGNNN!?
Chiyo bounces on Yuu's pelvis fairly intensively, with her busty beauteous boobs following suit.
Chiyo: Amazing... ♥ It's filling me up. Your dick's amazing, Yuu-kun. ♥
Yuu: Uah! Ah!...Agh!
The edging and vigorous mounting Chiyo imposes on Yuu numbs the latter's mind. So much so that the sensual sensations are constantly escalating to the point his body convulses, and his consciousness starts fading.
Chiyo: AH! ♥ AH! ♥ HA! ♥ AH! ♥ AHN! ♥ ♥ Amaziing. ♥ Ha. ♥ Ha. ♥ Your dick feels so good, Yuu-kun. ♥ Aha. ♥ Nfu. ♥ Ah~. ♥
Yuu: Ugh...Ugh! Ngh!
The vehemently sensual riding that Chiyo has given Yuu lasts for a favorable amount of time, with the former's thick thighs and alluring ass clapping and bouncing on the latter's groin from glans to base until Yuu speaks up out of desperation.
Yuu: Onee-chan! Onee-chan! I want to cum! Let me cum! Please!
Chiyo: Y-Yuu-kun...!!
Yuu: Nh! Nnh!
A moaningly weeping Chiyo slightly slows down her mounting pace and lowers her head to tongue-kiss a similarly weeping Yuu before stopping to speak up.
Chiyo: I love you. ♥ I looove you. ♥ You can't hold back anymore?
Yuu: Yeah.
Chiyo: You want to shoot out your stuff inside of me?
Yuu: Yes.
Chiyo: No matter what?
Yuu: Yes!
Chiyo licks Yuu's ear before she dotingly whispers to him.
Chiyo: Go right ahead. ♥
Chiyo moves up her upper body, continuing her vigorous ride with her supple breasts swinging at the same rate, and speaks to Yuu in her normal voice with a love-sickened expression, complete with heart-shaped pupils.
Chiyo: Shoot looots to your drippy semen inside Onee-chan's demon pussy! ♥ ♥ ♥
Yuu feels something akin to a laser piercing his mind as he is finally free of the edging spell Chiyo imposes on him. He quickly humps close to her cervix as his glans kiss it with every thrust, bringing much pleasure to her that is as otherworldly as herself.
Yuu: I'm cumming! I'm gonna cum, Onee-chan!
Chiyo: I'm cumming too! I'm cumming ♥ My little brother... ♥ A virgin... ♥ A human dick is making me cum! ♥ I'm cumming! ♥ I'm cummiiing~! ♥ ♥ ♥ ♥
Chiyo lovingly pushes her entire body with her soft boobs pressing to Yuu's face and her hands wrapping around his head and neck as the latter splurts his last batch of semen into her uterus.
Chiyo: AAAAAH! HYAAAH~! Haaaaa...It's coming inside...Oogh! ♥ Your semen... ♥ It's shooting right up to my womb... ♥ Amazing... ♥ Hyuh! ♥
After some clean-up from the rather rigorous sex Yuu and Chiyo have, Chiyo is back in her human form, wearing a tank top and shorts, waving a fan toward Yuu, who has his boxer replaced and sniffling from such an experience.
Chiyo: What's wrong, Yuu-kun? Are you crying?
Yuu: I started wondering if you were gonna go home now since the "contract" was fulfilled...
Chiyo: Fufu... ♥ My goodness, Yuu-kun...I'm a demon, you know? Do you really think that was enough for me? And besides...
Chiyo stands up and assumes her demonic form again to a nervous Yuu as she menacingly announces her epithet alongside another portion of the contract.
Chiyo: I am "The All-Mother Black Goat With a Thousand Young." In exchange for something very precious to you, I will grant you any wish.
Yuu: I want you to be my family...My Onee-chan!
A moment of awkward silence passes as Chiyo takes the time to process Yuu's abrupt response.
Chiyo: ...Is that really all you want?
Yuu quickly nods his head in response. Chiyo subsequently returns to her mundane human form with a slight blush.
Chiyo: When you said that, it made me a little happy.