Abject terror is a good start

"FUCK FUCK FUCK FUCK FUCK!!!" Layla screamed.

While she sat there in her bed, staring at the sizeable bulge under her covers, Layla found herself immobilized by panic.

"What the fuck is this?! What'd she do to me?!"

The answer was pretty obvious, but the shock hadn't totally worn off yet, so her blatant denial of the situation was understandable. Tentatively, she tapped the tentpole with her finger then quickly recoiled. Yep, she felt that. It was officially part of her.

Layla exhaled deeply and conjured the courage to lift up her blanket and gaze upon her addition. And as she did so, she bore witness to the chaos god's gift.

"Oh fuck me. FUCK! ME!"

The rigid tool stood at attention, throbbing with anticipation. Nestled beneath it were a pair of balls, each one comparable to an egg in size.

Layla rose from her bad, being careful not to jostle or pinch her newly grown manhood. She tip-toed to the long mirror on her wall and hoped to analyze the full extent of her changes.

"Shit. How am I supposed to deal with this?" Layla wondered.

She grabbed a measuring tape from one of her drawers and placed it alongside her shaft.

"Okay, Layla, calm down. Everything's fine. You're just panicking because you had a dream with a sex god and have a.... holy shit, it's half a foot?"

Her train of thought was disrupted by the discovery. Layla had always heard how the average male was about 5 inches long hard. And here she was, six and a half inches of cock in hand, easily creeping into the top percentile.

"Just great. She couldn't even give me something I can hide."

Of course, Layla was referring to Slaanesh. Who else could do something like this? Who else would do something like this?

As the girl turned to set aside the measuring tape, she noticed something else rather unusual.

"Hold on. What is going on back here?" she said, turning her back to the mirror while trying not to break her gaze.

It wasn't the girl's natural, dark brown hair, her tan skin, her amber eyes, her miraculously intact pussy, or meager B-cups that concerned her. Rather, it was the unusual mark which had appeared above her left thigh, around where it met the butt cheek.

"Where did you come from? Wait...."

Layla quickly realized how she recognized the symbol: a purple union of the male and female symbols, with a claw-like pincer at its end. Memories from her communion flashed back to her, the sensation of Slaanesh's palm striking her.

"Must be her way of marking me, showing off how I'm 'hers' now." Layla pondered.

Almost mimicking her patron, Layla extended a hand and slapped herself on the ass, right atop the amethyst mark. She paused for a moment, wondering why she did that. However, she seemed to draw some pleasure from it; the way her juicy ass rippled, and the firm, lewd clap follow that followed. Her attention turned to her still solid member. That little tease wasn't helping her situation.

"Well, I guess I was going to have to try it sooner or later." Layla said, submitting to her cock.

She took the throbbing rod in hand and began to pump, caressing the warm, veiny surface. The sensation was electric.

"Fuck." she cooed. "That's good. REALLY good. Does every guy feel like this?

With her free hand, she began rubbing her tits, pinching and pulling at her nipples. She closed her eyes and thought back to the night before; it was almost as if Slaanesh herself were still groping her.

Since she was still adjusting to her bodily changes, it didn't take long for Layla to cum. Spurts of white cum fired from her sensitive tip, splatting onto the mirror and dripping to the floor below. When it was over, her hand, thighs, cock, and reflection were slick with seed.

"Wow. That was amazing."

Maybe having a cock wasn't going to be a complete nightmare.