Sitting in a bar, a long shift behind you, as you nurse your beer (Lord knows you can afford another), a rather stunning blond woman with violet eyes sits down in the stool next to yours.
"Hey," she says, getting your attention, suddenly she looks almost disappointed "Aw, you're not adorably drunk yet." in an almost teasing tone.
"Drunk? In this economy?" you retort, this gets a giggle from the blond, she then leans forward a bit, she was wearing a red dress that showed off her assets but they weren't that big, almost a B cup, a man then gets her attention and she turns around, you then see she has more than enough in the back to make up for it (Let's say, 93 cms there), after she shoos the man away with the "I'm with him." excuse, she turns back to you as you lift your head so she doesn't catch you checking her out, her next words almost make you fall off your bar stool, "Wanna have sex?" she says casually, the man she just turned down turns and glares at you with such vitriol that you thought he was gonna jump you right there.
Before you could refuse she drags you out of the bar and into her car, almost 10 minutes later you've been stripped down to your underwear and thrown on to the bed and handcuffed to the post, even you realize this is NOT how this is supposed to go but hey, you're about to get laid so you can't complain.
"I'm going to slip into something comfortable, you stay right there." She says with a wink, "Really?" you say outload, gesturing to the cuffs keeping you in place, "were these necessary?" you ask, "Well, can't you getting cold feet." she says with a rather creepy smile as she enters the next room.
Oh, there's that bad feeling, you need to go, the cuffs look flimsy, maybe you can *ZAP*, "OW!" you whisper scream, now you can see something is wrong, just then the door to the other room opens and your "Date" walks in, she's now wearing a low cut black leotard with Crisscross strings going from her chest to her navel, what caught you was now she had horns and a tail. 
"I'm not into cospl.." You start, "Shut up." the woman states with a cold voice, "Food doesn't have an opinion." she says, her eyes violet now glowing, black feathered wings appeared on her back with a flash as she saunters toward you, "You ever bang a succubus?" she asks outright, you shake your head no, you understand the cuffs now, "It's an experience to DIE for." she says, now straddling you, your body unable to move.
"Let's hope that you're a good meal, the guy I ate yesterday was barely a snack." she almost growls out, Oh lord, she begins to pull off your underwear, "Hmm?" she hums in confusion, "You're not turned on?" she asks, almost disappointed, "Or do you have some kind of serious mental control?" she winks, "Well," You say flatly "You saying I'm your next meal is kind of a boner killer." She did not look amused.
Suddenly she smirks and her eyes flash, suddenly "You're little you" standing at attention, "There we g.." suddenly you gain an extra few inches in length and girth, "Well, that was new." she says genuinely surprised, you give her a look that screams "You're not really inspiring confidence here." but she shakes it off, pulls aside the bottom of her leotard and begins to mount you, the feeling is unreal, you try to fight the urge to bust your load right there, "Oh, so I don't need to even move....Hmm?" She suddenly looked confused.
She felt like she was getting tighter but the look on her face told a different story, you were growing inside her, this would have been a good thing normally (Though you've never had normal sex before), this was working against you as you now felt the uncontrollable urge to cum, she took notice and smirked a hungry smile, "Almost there." She cooed, she raised her hips slightly as you shake your head no, and she began riding you, sending a shock of pleasure throughout your body, this sent you over the edge.
The almost audible release came out like a torrent, nonstop and out of your control, it was flowing almost like a hose with no valve, she sat on you drinking it all in, face contorted with pleasure, then her face turned to confusion as she looked down at you, "It's not November yet, you know that right?" she says in a teasing voice but realizes something is wrong as your "Vitality" is still flowing into her, "Okay, that's enough!" she says panicked, trying not to raise her voice too high and draw attention to this room, she looked at you again in confusion as you don't look like you were shrivelling up like her other victims but still looked like you had more to give, she suddenly twitched as her body throbbed, "What's going on?!" She tries to pull herself off of you to no avail.
She tries to look you in the face to ask you to stop again and sees you're wide eyes staring at her, more specifically, staring at her chest, in the demons panic at the situation she hadn't realized her breasts had swollen from a B cup to a D cup now bulging out of the leotards top as it tried to stretch to accommodate the sudden growth, "Well," you said between gasps "all that vitality has to go somewhere." She gives you an unamused look, well, tries to but the pleasure is starting to get to her as her body throbs again, her breasts start quickly growing as the fabric stretches and groans as it's reaching its limit.
"Is it wrong that I'm enjoying this?" You ask, "I wanna say yes!" The succubus retorts but can't bring herself to say what's really going through her head, fondling her now F cup and still growing breasts, the strings of the leotard's chest starting to hit their limit.
"More," She gasps out, you look confused despite the situation, "MORE!" she now yells, any remaining control you have is gone as your body responds, a massive amount of you "Vitality" quickly floods into her, her eyes roll back in her head as her body starts throbbing from the influx, her breasts suddenly swell to J-Cups as she reaches her climax and her leotard, despite its valiant efforts, fails to hold back her new melons and a loud snapping can be heard as her breasts bounce free.
Just then, it stopped, the succubus was left panting as you went flaccid and she fell to your side, just then the cuffs holding your arm fell off but you're too exhausted to even escape.
Almost purring, the succubus looks at you with a look of contentment, "Name's Mellia Lux by the way, but my friends call me Mel" you give her your name.
Before you drift off you decide to have the last word, "Wanna do that again." Mel glares at you and smacks you with her tail, you let out a sleepy "ow" and you both drift off to sleep.
*Next morning*
"So, you're telling me there was some kind of mana resonance between us?" Mel asked someone over the phone looking annoyed as you wake up, "And this caused an effect that caused growth in both of us?", you look at her and you see she was wearing an oversized shirt but she heads into the next room before you can greet her, as you get up and stretch a loud "He's my WHAT?!!!" snaps you fully awake.
"So, it turns out our mana is really compatible, this caused, well, last night." Mel explained vaguely then taking a long chug of her coffee, taking a look at her short you can tell her chest had shrunk down, "They've shrunk down to E cups by the way." Mel says almost as if she could read your mind, "I'm going to have to get more stretchable leotards, unless you wanna see them grow bare?" she says almost seductively, you take a drink of coffee but you almost spit it out when your mind registers what she said, "What?" you say flatly confused, "Well, it seems such a pairing is once in a demon's lifetime, so you're mine and I'm yours now." She walks up and kisses you, "You were the most filling meal I had and it felt really good growing like that." She tells you blushing, "But we'll also have normal sex too, I just won't turn on the Vitality Drain." she says in an as a matter of fact tone.
"So we'll be dating?" You say monotone, the news still sinking in, "Mhm." Mel nods, "If you want to put it that way, more like a contract.", you nod blankly, she notices "You okay?" she asks "Yeah," you say, "It's just, I'm dating a succubus now." she smiles mischievously "Any plans for today?" You look at her getting where she's going with this, "No and tomorrow too." you say with a hungry smirk, she saunters up to you and with a wide smile and a whip of her tail around your leg she says "Wanna have sex?".
With that your strange relationship with a succubus begins.
