Yes

Your sense departed you a long while ago leaving nothing in your mind but a haze of sex. Your head is spinning, your desire is driving you mad. Greed thick in your voice you mutter "yes."

She smiles wickedly and drapes her arms over your shoulders. She pulls herself onto your lap. Her legs curl around your back. She cranes her neck until her plump lips press against your ear. Her mouth is soft and warm. Her voice low and feral "You need to tell me you're sure. Then there's no going back." You can't hear her. You can't think. Your mouth hangs slack. Waves of heat rolled off her, getting faster and faster. Each wave making you want her more. The denim of her jeans pressed taught against his cock. You lift your arms to bring her closer. She keeps herself against you but pushes your arms down. She lustily hissed "Not yet. First tell me your sure."

Lost in lust you ignore her. You begin thrusting, rubbing your cock against her. You abruptly stop when you feel her sucking your earlobe. You slump onto her as she gently sucks. The warmth of her mouth and the soft noises she was making made you so horny. You can't remember where you are, or even who you are. All you knew was lust for her. Your head lolled to the side and every part of your being was focused on her. a small moan escapes your lips as she stops sucking and you hear her growl "Tell. Me. You're. Sure." Your eyes are half lidded and speech slurred as you breathed "I'm sure"

As soon as the words leave your mouth she hops off you. Your mind immediately clears. You look at her confused. "did you just... Brainwash me?" She smiles and said "sure did!"
"Why? I was about to say yes..."She shrugs at you and says "Dude, I'm a demon. We do weird shit just to fuck with people... It's fun"
"But doesn't you doing that mean you **** me to agree?"
"No, I just made you horny. You could have resisted if you wanted to. doesn't matter anyway, you agreed. Lets get down to business."

You don't think her logic is sound, but you also don't care. Having those kind of powers seems fucking awesome. "okay, do I have to sign something or what?" She quickly shook her head "It's more involved than that." She waves a hand at her desk and it disappears. on the floor underneath the desk was a red pentagram. She gestures you over to it. Excited you make your way to the middle of the pentagram quickly.

"Okay, now kneel and repeat after me." You kneel.

"I willingly pledge myself to darkness and I willingly give up my humanity" After you repeat her words back to her she nods. A jagged stone bowl filled with a black liquid appeared in front of you. "drink the entire bowl" she command.

You grab the bowl and eagerly down its contents. Whatever was in the bowl tasted foul but you finish quickly enough. As you gulp down the liquid you start to tingle. soon that tingle turns into a burning that engulfed your body. Your muscles get bigger, bones longer. You now stand over six feet tall and every inch of you is hulking an muscle bound. As you grow your skin becomes tinted red. You raise a hand to your forehead and find two curving horns about eight inches long. After a few more moments the burning is gone completely. You smile as you take in your new body. You're massive and every inch of you is muscle. Your hands are huge and meaty, you think you could easily crush a skull in your hand. your fore arms are equally impressive your biceps are the size of watermelons. Your entire body has been similarly modified. You're most please with your cock. what was a respectable six inches when fully hard is now massive. your cock hangs heavily between your legs. it's as long and thick around as your old arm! what's more is your not even hard. You can't wait to try your new self out. you look at the demon who had converted you. you now tower over her, she barely comes up to your nipples. She looks at you approvingly and says "good now, the fun part."

she steps toward you while saying "you're no longer physically human. Now we have to change you mentally." when she lays her hand on you, you both disappear.