Having disposed of the intruder in her lab, Fionna made her way through the hallways towards the exit, intent on bringing her new abilities to bear upon the outside world. Slithering out the doors, she emerged on the twilit streets on the edge of the city she called home. However, she didn't need to go far before a monstergirl called out to her from up above her back.
"My, I haven't seen anyone like you before."
Turning around and looking up, Fionna saw what she could only describe as an ascended succubus queen. The purple-skinned demoness positively oozed beauty, power and confidence, between her four impressive breasts, her three yard-long dicks standing at attention above her dripping pussy, and the smug grins displayed on all three of her heads. Two large bat-like wings kept her aloft above the streets, and her long, spaded tail kept waving around, occasionally wrapping around one of her long, toned legs. To round out the package, the succubus had three pairs of arms, and medium-sized horns protruded up from each of her heads.
The regal demoness continued, "I can sense something dangerous about you, a sort of latent and growing power... But that danger intoxicates me, makes me want to just dominate you even more." One of her heads licked her lips as she said the word "dominate", while another smiled and chuckled.
Fionna saw an oppprtunity to turn the monstergirl's pride to her own advantage, and decided to play along. "I'm flattered that a woman of your caliber would take interest in me," the scientist started. "If you want a taste of that 'latent power' you mentioned, I'm free all evening. I promise you won't be disappointed."
The demoness' lips curled into a devilish smile upon hearing Fionna's words. "A promise you say? You better pray you live up to your words, I show no mercy to those who disappoint me," she replied.
"Shall we go somewhere more private?" the scientist queried.
Without warning, the succubus swooped down, grabbing Fionna with her six arms and lifted her heavy lamia body off the ground effortlessly. She then flew to an abandoned mansion by the edge of the park, diving through a hole in the roof and landing in the master bedroom. Pinning the lamia to the bed using two of her arms, the demoness began aggressively kissing her while using another pair of arms to grope her breasts.
Speaking from one of her other heads, the succubus remarked, "mmm, you're a great kisser. This is my first time seeing a lamia with alraune traits. Got any other surprises?" With some effort, Fionna broke the kiss so she could answer. "I might. My vagina is actually quite flexible. I'd bet you I could fit you entirely inside it," Fionna added with a wink. "Unbirth? Ooh, kinky," the demoness replied, "I'm game to try. Be ready for some punishment if you fail though."
"I won't."
With that, Fionna reached down to her snake-vagina, and pulled it wide open in a way to invite the succubus inside. Halfway cautiously, halfway eager, she placed one foot inside the pussy, immediately feeling it sinking in. Following with the other foot, she quickly found herself sliding further into Fionna's hole, already up to her kneecaps.
"Mm, this feels surprisingly good," the succubus moaned as she began fondling all four of her breasts.
"For you and me both!" Fionna gasped as she allowed her partner to sink further in.
By the time the demoness sunk down to her bellybutton, she noticed something odd. "You know, I can't even feel my feet now, it's almost like they've... dissolved?" A few moments passed until she registered what she just said. As it hit her, she suddenly began squirming. However, Fionna had no intention of letting her prey escape, and began pushing the succubus further in with her arms. Try as she might, the demoness couldn't escape, and shrieked as her heads sank into Fionna's vagina.
After a few moments, the scientist could feel her prey's body finished dissolving, and sensed the new DNA available to her. Focusing, she grew two new pairs of arms in addition to her old ones, and felt them swell with demonic strength. Additionally, she grew a new pair of breasts below her originals, then let them grow to twice their old size as she removed the flowers that replaced her nipples in favor of regular nips. She then sprouted three dicks above her vagina, and grew out a pair of wings from her back in addition to a spaded tail above where her human torso met her snake tail. Finally, she let horns grow from her skull, parting the vine hair on her head, then gasped in triplicate as her head split in three. It took a moment to get used to having triple the sensory input, but she quickly grew to enjoy the advantages that multiple heads afforded her. Slithering over to a large mirror, Fionna narcissistically appreciated her new look, giving herself a double kiss on the cheeks before turning her thoughts towards her next move.