"OK!" Derrick grinned. "I'll be a cerebus!"
Hebe nodded and helped him out of the basin. "Can you pretend to be a dog for me? Get on all fours and only bark?"
Derrick dropped to his hands and knees and woofed.
"Excellent! Now, remember to just bark. You are not a boy any more, you are a Cerebus."
He liked this game. Derrick started crawling around the basin, making barking noises. As he did, the idea of becoming a monster dog got clearer in his head, and he started to get more into the part. There was movement and a strong odor that made him turn towards the back of the room, and he froze. A very tall, dark man dressed in black came out from the curtains. He was pale and slender, but radiated power. He smiled when he saw Derrick and patted his leg. "Come here, boy. Be a good dog."
Derrick crawled over, letting his tongue hang out. He looked up and grinned, then woofed.
"Excellent. Let's see if we can get you fixed up here so it isn't an act anymore." He reached down and started to scratch Derrick's head. It felt good, and sent a ripple of warmth down his neck and back, ending in his toes and fingers. "Already sprouting fur! You will be such a good dog!"
He kept up the petting, and the waves of heat pulsed steadily. Derrick shifted as his posture got awkward, and his face felt swollen. The man in black seemed smaller, no longer stooping to scratch Derrick's head. And Derrick's fake woofs got deeper and more realistic. He caught part of his reflection in the mirrors and turned his head to look. The image was startling - the man in black was petting he head of a thing that was more black mastiff than human boy - it was him! He watched in fascination as his body and legs shifted, the coarse black fur spread over his skin, and his nose and mouth pushed out into a muzzle. And still he got bigger!
Derrick whimpered suddenly as a throb of pain shot through his skull. The man in black patted him and said "It will only hurt for a short time, and you will get a little confused. Don't worry. You will be fine."
A whine emerged from his throat despite the reassurances, and he shook his head in sudden agony that faded almost instantly. Only now he was bewildered by too many inputs. There were too many views, too many sources of scents, too many sounds. In the mirror, he saw the massive dog's head dividing, each split forming a new and identical dog's head. His mind was disconnected, thoughts confused and unfocused.
The man in black chuckled. "Having a hard time thinking? Don't worry, my canine friend. Three heads will soon have three identities. Sadly, none of them will be human - Cerebus is a three-headed dog. You are going to be a genuine monster forever."
Derrick felt a sudden panic as he realized what the man was saying - but it was already too late. As the three head completed forming, his intelligence split, awareness divided, and Derrick the boy became Cerebus 1, 2, and 3. Each head was aware, each head its own being, yet they coordinated together for the one body. The monster shook its heads as it continued to grow, soon dwarfing the man. They snarled in unison, drawing back black lips to reveal huge fangs. The last traces of Derrick's memories faded as new instincts and memories of life as three-headed monster filled all three heads.
When the beast stopped growing, it was eight feet tall, probably weighed more than two tons, and looked and smelled like a creature from Hell. Which, in fact, it was. Hades patted the beast's shoulder in approval, looking up at the slavering, vicious monster. "Good boy!" The praise from its master made all three heads pant happily.
Hades looked at Hebe. "So, I trust I do not have to return him?"
She shook her head. "No. You have a guard beast as long as you want him. I'll handle the family."