Posted by this anon on /d/ a few years back.
> You're an Adventurer, walking along a secluded forest trail with a Succubus
> You rescued her from death at the Hands of an angry Paladin, and now you're leading her to safety
> Stopping at a fork in the road, you turn to her
"The path this way should lead you back towards Monster Territory. Stick to the side roads, and you should avoid most trouble with other Heroes."
> She takes your hand into hers with a warm smile
> "I appreciate everything you've done for me. Isn't there any way I can repay you?"
> You shake your head
"That won't be necessary. Just try to stay safe in the future."
> She frowns, unsatisfied with your answer
> After pausing for thought for a moment, she grins
> "Perhaps I could offer you some of my Demon Magic?"
> Your eyes widen
"You mean that evil stuff that drives men insane? No thanks."
> Her grin widens
> "No, that's the beauty of what I'm offering here! It isn't the Magic itself that drives Men mad, it's the Cursed Knowledge that leads to it! I'm offering you the chance to use it while remaining safely ignorant!"
"What?"
> "Think of it like using an enchanted scroll. You don't know anything about the spell, or how it works, but the scroll lets you cast it anyway!"
> ...
> Demon magic is pretty powerful
> It could be useful
"How exactly would this work? Can you make me Demon Scrolls or something?"
> "No, silly, you can't transcribe Cursed Runes."
"Then how can you-"
> "Like this!"
> A flash of black flame suddenly consumes her body, leaving nothing behind but her clothes
"What the fuck!? ...hello?"
> "I'm down here!"
> Her voice resonates inside your mind, drawing your attention to the pile of clothes as a tiny, naked Succubus climbs out of them
> "What do you think? Cool, right?"
"Why are you small now? How exactly is this supposed to help me?"
> "Pick me up real quick"
> You reach down and scoop her up into your hand
"Okay, now what?"
> "Now take out your dick."

> The order takes you by surprise
> You had expected her to make some kind of move, being a Succubus and all, but you didn't expect herself to shrink herself down to such a small and powerless state first
"Um, o-okay..."
> "You might want to make yourself comfortable while you're at it."
> Nervously you nod, and head off the trail to go sit under a large tree
> Your free hand quickly undoes your belt and pants as the little Succubus watches eagerly from the palm of your other hand
> Once you've proverbially freed the beast, she whistles appreciatively
> "Nice cock. Although, I guess any size is going to seem impressive when you're tiny like me."
> Before you can respond, she jumps from your hand down onto your semi-flaccid dick
> Thankfully she's light enough for that to not hurt
> She's only about an inch tall, allowing your cock to totally dwarf her
> Despite the disparity, she expertly rubs, nuzzles and licks your manhood
> "Do you like that?"
> Your rapidly stiffening erection is the only answer she needs
> She holds on tight as your cock begins to stand tall, and then begins to focus her affection on your cockslit
> Precum builds inside of you, until it oozes from the tip
> "There we go, that's what we're looking for."
> You yelp in surprise when she plunges her arm down your urethra
> "Nice and lubricated...does that feel nice?"
> You nod
"Good."
> She climbs up on top of the head of your penis
> One foot goes in, and then other
> You suck in a breath as her feet push deeper into your urethra
> This is so strange, why isn't she doing this with her arms?
> As you watch (and feel) her pushing her knees past the opening of your urethra, your stomach turns as you realize that she isn't merely stimulating you, she's going in
"Wa-wait, what the fuck are you doing?"
> "You'll see! You'll really like this!"
> You reach down to stop her, but she quickly sends a pulse of dark magic through your body that paralyzes you

> You're cursing yourself for trusting a demon when she calls out to you
> "You wanted me to help you use Demon Magic, right? Well this is how we're going to do it."
> When she turned tiny and started wrestling with your cock, you had kinda forgotten why you were doing this...
> "If you're having second thoughts, that's fine. But if you want my power, you have to let me do my thing. Okay?"
> Surprisingly, she releases her hold on you, allowing you to move freely once more
> She resumes her descent into your cock, wiggling her hips enticingly to inch her way further in until she's been engulfed up to the waist
> Tucking her hands inside, she looks up to you
> "Alright, things are about to get a little weird. You ready?"
> No, but you nod your head anyway
> With a pulse of Black Magic, your cock comes alive
> It hungrily begins to devour her, sucking her in up to her breasts, and then up to her neck
> "See ya~"
> With a powerful gulp, your cock drags her fully inside, quickly sucking her several inches deep
> You're panting and gripping fistfuls of grass at the sensation
> What the fuck is going on? What is her goal?
> You reach down and caress the underside of your cock with your fingertips
> There's a prominent bulge in your urethra that gently pushes back against your touch
> Oh God
> You moan as another gulp drags her even further down
> The bulge begins to descend past the base of your cock, disappearing into your body
> Your whole body convulses in pleasure and shock as she is squeezed into some hyper-sensitive place within you
> "And here we are! The prostate!"
> Her voice resonates inside your head once more
> Prostate?
> She's...inside your prostate?
> She wiggles to adjust herself inside you, and you moan like a whore
> "Does that feel good?"
"W...why are you doing this?"
> "I'll explain in a minute. For now, just relax, and let me take care of you~"
> Her movements are slow and gentle as she begins to massage your prostate from within

> Your breath is shaky, and you quiver like a newborn chick as you experience the greatest pleasure you've ever felt
> Instinctually, you begin weakly thrusting your hips into the air
> "You like that sweetie?"
> You can feel her drag her tongue along your inner walls, which drives you over the edge
> Your vision goes white, and you cry out to the heavens in ecstasy as a geyser of cum erupts from your cock
> It splatters against your stomach and chest, but you are too exhausted to care
> Your blurred vision refocuses, and you glance at the messy aftermath
> You don't see her
> With an orgasm that powerful, you would have expected to have shot her out
> A little squirm from within confirms that she's still inside you, having not budged an inch
> "Oh wow, I almost thought you were gonna squish me for a second there! You've got a real strong prostate on you!"
"Why...why did you-"
> "I'm a Succubus. What do Succubi eat?"
> The cum splattered all over and around you fizzles away into a wispy trail of black energy that snakes it's way back down your urethra in a way that's not entirely unpleasant
> "Such a delicious orgasm~"
> You take a few moments to catch your breath as the Succubus remains mercifully calm inside you
"But...why did you have to be inside of me for that? Couldn't you have just given me a blowjob if you wanted to feed on me?"
> "I'm not just in here to feed. Stand up."
> It takes a minute for your weak legs to respond, but you manage to pull yourself up onto your feet
> "We'll start with something simple. Just throw a fireball."
> You start to argue that you don't know how, but realize that you actually do
> You concentrate on forming a ball of fire in your hand, and jolt at the strange sensation inside of you
"H-hey! What the hell was that?"
> "That wasn't me, babe. You're channeling some pretty powerful Dark Energies through your prostate right now, so you're going to have to try and get used to that feeling."

> You refocus on the task of forming the fireball, trying desperately to ignoring the hot, tingling sensation from inside you
> A tiny little speck of flame forms in your hand, which is good enough for now
> You toss it against a nearby tree trunk, causing a bright flash and singeing the bark
> "There you go! That wasn't so hard, now was it?"
> Maybe not in the sense of difficulty, but your dick is rock hard
"What is this? What is happening?"
> "You're channeling my magic. As long as I'm inside of you, you can use almost any of my spells! I'll only let you use a few basic ones for now, until you get used to me."
"Why do you have to be inside me for that?"
> "How can I put this in a way you'd understand...think of me as a piece of equipment that you have equipped to your 'prostate' slot."
> That actually does sort of make sense
> Like a magic ring, or something
"This is so weird. You can go ahead and come on out, and we can practice more after I've had time to rest."
> "Nope!"
"Huh? N-no?"
> "The way I see it, staying inside of you is a win-win. I get protection from other heroes and as many orgasms as I can eat, and you get access to powerful and forbidden magic, as well as those orgasms."
> You'll die
> Just the weight of her laying still inside your prostate is intense
"You can't stay in there! It feels-I can't handle that sensation all day! It'll drive me mad!"
> "Don't worry, I'm not going to stay on top of your g-spot all day."
"...but I thought you just said-"
> Your voice is silenced by the incredible sensation of her moving once again
> She pushes herself deeper still, forcing herself through some sort of narrow aperture below your prostate
> The pressure is intense; if it wasn't for her magic, you'd probably be in pain right now rather than mild discomfort
> "Ngh...don't worry babe, you'll start to like this too once you...get used to it..."
> She struggles her way down, and you start to feel her weight more and more as she makes progress

> Your left testicle twitches, and then begins to slowly swell as she pushes herself inside
> The last of her finally emerges from your narrow duct, fully encasing her in your nut
> It feels utterly bizzare, but at least it isn't as distracting as when she was in your prostate
> "Ah, finally. Home sweet home, eh?"
> She wiggles around inside, pressing against you as she makes herself comfortable
> You furrow your brow as you stare at your ballsack
"You're...okay in there? It isn't too tight?"
> "Oh, it's skin-tight. I'm practically vacuum-packed in nutmeat in here. I can feel billions of microscopic little sperm cells wriggling and squirming against me, and they flood into my mouth if I dare open it."
"That sounds horrible."
> "It's practically heaven to a subby little succubus like me~"
> As long as she's happy...
> It feels weird buttoning up your pants after what just happened
> There's almost a sense of finality to it
"How long do you plan on staying in there?"
> "As long as we're traveling together. I'll stay in your ballsack most of the time, and I'll move up into your prostate to feed, or just if you get horny."
> You buckle your belt and dust off your pants
> Looking down at your crotch, it's a little surreal that there's no evidence of your little passenger
> Returning to the road, you stop at the fork you had originally led the succubus to
> For whatever reason, you decide to head down the branch leading to Monster Territory
> Shouldn't be too dangerous with Demon Magic at your disposal...