Mary needs to earn it

Mara grins as she pulls her tail out of Mary's needy pussy. "Noooo..." she whimpers. "Don't worry, you will become soon, but you need to work for it. Follow me." In an instant, the car door is open and succubus-Mara is dashing at the young man outside. He had been very turned on by the lesbian act he had been spying on, but seeing a succubus charge at him is enough to make him try to run for it. Not that it helps, Mara catches him quickly and drags/carries him struggling back to the car as her transforming is still getting out. "I know you liked my pussy and tail, but there's no substitute for a real cock!" Mary licks the black cum from her lips as her gaze goes from her former friend to the young man and back.

"Please don't! I'm begging you!" the man whines, but Mara shushes him. "Just relax, let it happen..." Mary kneels before him and without a word goes for his dick. Taking it in her mouth and humming happily as she sucks him until he is hard again. She then wraps her growing breasts around it and gives him a titfuck. Mary is corrupted beyond hope by now and with the daemonic juices from her mouth infecting the man, his cock begins to show signs of corruption as well as it swells much thicker and longer than it was when he was when Mara caught him spying on them. "Mmm... Yummy. This is going to feel so good inside me... I hope you enjoyed my final blowjob as a human, now you get to have a last try at my pussy too before I become a succubus." Mary lays back over the hood of the car and spreads her legs. Mara guides the man inside, Mary looks like she is about to cum even from being filled by the cock for the first time. "If you do a good job fucking her, we might even spare you..." Mara whispers in the man's ear and knowing he has no other choice, he gets to work.

Mara watches from the side while fucking herself with her tail, for a while. The young man certainly seems to be doing his best, with Mary moaning and shivering each time he pushes deep inside her. Telltale signs of their progressing transformations are showing up, but without a succubus/incubus actively participating, it appears to be slower than normally. More time for them to enjoy it at least, Mara thinks happily. Finally growing too hungry and horny, she again slides closer behind the coupling humans and pushes the man's head down onto Mary who pulls him in and makes out with him. Mara guides her tail underneath the man and into Mary's asshole. It goes in easily enough, a clear sign that Mary is almost done. And with the tail and cock both penetrating her, she finally climaxes. Her body shakes uncontrollably and even though the young man tries to break free, she smothers his face between her now massive breasts. His protests are muffled, but would not have been audible over Mary's orgasmic screams anyway. And as Mary's succubus cunt grips on him hard, he stops trying to escape and again is fucking her, this time desperately trying to cum as the unimaginable pleasure threatens to drive him mad without the release of orgasm.

Now, there is only the question if the two new and hungry succubi's would actually keep their word and spare him, or if they should feast on him together like good friends they were.