You take her by yourself, and Evelyn joins in after.

You move forward, but Evelyn hangs back.
"I'll just watch I think. Then join for a nice fuck after." Her hands are already roaming to her wet vulva as she says this. Fine by you. You approach the desk, then shove it aside with such **** it breaks in half. Diane screams and covers herself in vain. You step forward, grabbing her wrists so as to get a good look at her face. Time to try something new. You stare deeply into her eyes, compelling her to your will. At first you see nothing but fear, but quickly her look softens, first into a blank stare, then into sleepy happiness.
"Mmm Linus." She says, accepting your embrace. "I always did want to fuck you." She pushes you back lightly and you let her. She begins to strip, swaying her nubile body to imaginary music. First, she pulls up her top, lifting her boobs with it until the last second, where they drop performing a lovely bounce. Then, gyrating and thrusting, she pulls her shorts down, revealing her wet cunt. Her hands run over her body as she approaches you.
"As you can see, I'm a massive slut. A slut who neeeds your cum." She kneels in-front of you, and undoes your pants letting your massive erect cock hang out. She giggles with delight and runs her mouth and tongue along it. Her warmth feels exquisite against your cold, hard skin. She takes your cock in her mouth, never losing eye contact. She pulls it and you feel the tip graze the back of her throat. She gags but continues to press back and forward, her tits jiggling with each vigorous thrust. You begin to push in as well, picking up the pace until eventually you feel climax coming. She realises this too and, being the loyal servant that she is, she holds your penis in her mouth ready. You cum violently, your rocking penis shaking her head, your cum spilling down her throat. She holds her breath, swallowing as much as she can but eventually she has to pull away, your supernaturally enhanced load too much for her mortal body. She spurts cum onto the floor, a large amount dribbling down her front. You see a droplet hanging from one nipple. She looks up at you, presenting the large amount of white sperm in her mouth, then she closes and focuses as she swallows the rest. Finally she shows you a clean tongue, clearly very pleased with herself. You reward her with your cock down her throat once again, but this time you grab her ankle, and swivel her around so your form a standing 69 position. Excited by the new move she begins her fellatio afresh. You plunge your tongue into her vagina, eliciting a quiver from the woman. This position is unusual for you as you find yourself pleasuring her clit with the underside of your tongue and lower teeth, but you have no trouble. You continue to suck, lick and graze her cunt as you both approach climax. You introduce more teeth, the tips of your fangs brushing her inner vulva. From below you hear Diane say as best she can: "Please put it in! Fuck me with your fangs! Make me like you!" It would be rude not to oblige. You slide your fangs in easily, the blood welling up and dripping down to her clit where you lap it up hungrily. Diane screams and orgasms, her teeth digging into your cock and causing your final explosion. The blood gushes as she shakes from orgasm after orgasm. Then you feel Evelyn taking her from you. Your penis still remains inside her cum filled mouth but now Evelyn is holding onto Diane's buttocks. She looks up at you smiling misceviously as you spy what appears to be a strap-on tied between her legs.
"You like it?" She says as she enters the girl with the massive sextoy. "I found it in our little slut's desk." She begins to pound her, and you grin and follow suit holding onto the back of Diane's head as you throat fuck her. You both watch as her body transforms, breasts becoming even larger and firmer, ass becoming firm and round, as her muscles tighten and grow in all the right places. You feel her fangs grow in you mouth, caging your cock as you thrust deep into her. Her now white blonde hair dangles down elegantly, swaying gently with each thrust. Finally, her neon blue eyes open, and she looks up at you, nothing but lust and hunger in her eyes. She grabs at your buttocks and pulls you in harder, all the while writhing impossibly, and grinding against Evelyn. Her tongue wraps itself around your shaft and you feel yourself nearing completion. You come, pulling your cock from her mouth and spraying the massive load over her twisting, orgasmic body. Some of it hits Evelyn's abdomen who has also reached climax. You all collapse to the floor, utterly relieved. Diane reaches up and strokes your face, licking some cum off of her lips, as Evelyn laps at a strand on her tits.
"Thank you for the best fuck of my life..." You smile wide in response, your penis comfortably nestled between her slender legs.