"You want more seed, slave?" the doppelgänger chuckled, as he pounded away at Brian's drow pussy.
"Oh yes master!" Brian moaned, unable to help himself. This treacherous female drow body was overwhelming him. It was built for pleasure, and left him powerless to resist.
The doppelgänger laughed again, and began pumping his cock in hard, fucking Brian roughly, using his shapeshifting abilities as he went. He grew, his body swelling even larger, his already thick gray muscles becoming freakishly large, his big cock swelling inside Brian's pussy, stretching his drow cunt. Brian writhed with pain and pleasure, still tied to the rack, as his new master grew large and freakish. The doppelgänger rammed his oversized fuckrod up Brian hard, becoming rough. Brian moaned, not sure which was greater: the agony or the ecstasy of being pummeled by such a huge dick. But as he lay there, writhing, Brian realized that his pussy was adapting, growing to accommodate his master's huge invader. In fact, his whole body was slowly becoming... less rigid... he realized.
The doppelgänger's breaths were coming quick and heavy now, as the huge beast rammed himself up Brian. "I- I can change..." Brian murmured in wonder. He took a deep breath, and felt his chest swell, his already large drow breasts growing even larger, filling with soft, sexy tissue. Above him, his master smiled, using his own abilities to lengthen his tongue so that he could lick Brian's swollen mammaries, driving Brian wild with lust. "Sssoon..." the huge freak hissed, "You will be my thrall!" With a harsh chuckle, he pounded away at Brian's pussy even harder, using his huge body to ram his huge cock up Brian, as rough a fuck as the drow had ever known. The doppelgänger's thrusts came harder, and his breaths faster and deeper, and with a cry of lust, he came, pumping even more seed into Brian, setting the drow afire with pleasure, even as Brian could feel himself changing, becoming even less rigid.