"Centaur... Proud and strong. You shall be the first to gain this gift."
The voice disappears as you feel your feet tingle. You close your eyes as you feel your body stretch, new mass being added to your frame. You look down and see fur covering your expanding body, another pair of legs being added to your frame as you gain the lower body of a horse. Your hand rubs through your chest, which feels rugged, covered in a new layer of mainly hair and muscle. Eventually, You feel your hair touch your shoulders as it grows, becoming long and noble.
Your new horsecock twitches as your transformation ends, you shoot some of your seed across the cell, and directly onto the face of a shocked prisoner. The wizard's voice fills the room again. "My new child is as virile as I thought, pleasure the transforming human, so the others may witness what will be their fate..."
The cell doors open as you walk towards the human. He stands still and moans, his legs undergoing the same transformation yours took. You kneel and lower yourself, placing your lips across his enlarging member. You kiss and suck at the head, your mouth stretching as you feel the prick becoming larger and larger. Eventually, his oversized horse cock is stuffed down your throat, for a human this would be painful, but your centaur body is made to fit the cocks of your brothers. You eagerly await your prize, eyes closed as you eventually feel the release of his seed. You drink your fill, then open your eyes to find yourself in a forest.
You stand next to your brother, watching and waiting. Eventually another centaur approaches, his blonde hair in a long ponytail. "Welcome my children, I transformed you into what you are now."
You and the other centaur bow to this man, he chuckles "No formalities yet my children, there are humans to change. Stuff your horsecocks down their throat so they might join us in this life!"
You nod to your new father, then trot along, looking for humans within this forest.