You step forward.
The dragon nods. "Very well, kobolds, take the rest of these humans and put them to work. I'm not to be disturbed until I call for you." As the humans are led from the cavern the dragon lowers his face in front of you. You're pretty sure he could snap you up and swallow you in one bite. "Hm, brave little human, and you can think for yourself instead of following along. I might have use for you."
Suddenly he rears up and his entire body is engulfed in flames, though he doesn't seem to be burning, instead he's shrinking. You can see his silhouette inside the flames shedding mass as he reduces to something close to your size. Closer.. but not quite. He steps from the flames, now humanoid in shape. He's large and muscular, still towering over you at eight feet tall, and built like the strongest warrior you have ever seen. His body is still covered in crimson scales, and his head is not much altered, still draconic with flames licking from the corners of his mouth. His wings spread behind him, wide and powerful, and his tail slithers through the piled of gold behind him as he approaches you and legs and feet that are still distinctly draconian. He's also sporting a rather obvious erection, the skin a bright red, ridged along it's length, and about as long as your arm, and thick as your leg.
"It's been a while since I've taken a humanoid form. You should be honored." His voice is still a deep rumble, powerful and strong, and leaving no doubt as to who is in charge. At a loss for what to do you kneel in front of him. "Do you wish to serve me?"
"I.. I do." You say, knowing any other answer might result you burning to a crisp.
The dragon laughs. "I can almost believe you. But if I gave you the choice to serve me as a kobold, or as something greater..?"
"I.. I would rather not be a kobold, lord... dragon." You keep your eyes downcast.
"Kaestryx. Do not try to hard, it is difficult for human mouths to form the proper sounds. Give me your oath, you will serve." A knife fallsat your feet, an obsidian blade with a golden hilt and a giant ruby as the pommel. The ruby seems to glow. "Cut your hand and spill your blood with the knife and swear to serve. The kobolds are too stupid to disobey, but I will not give you power without this promise."
You pick up the knife and slice across your hand. Your flesh sizzles and burns as the blade parts it, and you clamp down on a scream of pain, intent not to show weakness before this dragon. Blood wells forth and you let it drip to the ground. "I swear on my blood to serve you, Lord Kaestryx, faithfully and to the best of my ability."
You finally look up and swear you see the dragon grinning. "Very good." He takes a deep breath in and exhales a plume of flame that engulfs you. You expect to burn, but instead you feel more alive than every before. Your clothes burn to ash, falling off around you, but the cut in your hand heals. All the little scrapes and bruises you've taken since your capture heal. You breathe in and feel the fire filling your lungs.
Your body begins to swell. Lean muscles, become larger as power courses through you. Your chest suddenly pushes out with strong, firm pectorals, your arms thickening to the size your legs once were as your legs practically double in size. Your whole body courses with new found strength as the fire floods your bones, pushing them to grow bigger, thicker, and heavier. Soon Kaestryx doesn't loom over you quite so large. You aren't nearly his size, and his muscles still outstrip yours, but kneeling your head is now level with his stomach rather than his thighs.
"You've taken in my fire, now human, take in my seed." You reach up and take his cock in your hands. It feels hot in your hands, but not as hot as the fire you can feel nestling in your chest. Opening your mouth, you wrap your lips around his head. The taste of his pre is warm and spicy. You run your tongue along the length of his shaft, tasting that unique flavor. Your shoulders and chest begin to itch as red scales push through your skin. Soft and supple for the moment, but you know they will soon harden. This skin on your forehead parts as nubs of horns start growing from your skull. Every drop you drink from his cock causes you to shed a little more of your humanity.
You nails begin to harden into dark ebony claws, your face pushing forward as your mouth fills with sharp teeth. Too slow.. it's too slow. You turn around and present your ass to Kaestryx. He laughs eagerly. "Oh, you are the good servant." You feel his clawed hands grip your hips, and then his hot, rigid cock penetrating deep inside of you. As a human in would have split you in half, but now it feels like it fills you perfectly. Like you had hoped, it seems to speed your change along as your dragon master thrusts himself in and out of you. Your legs begins to reform, scales covering them as they take on a more draconic shape, much like your master's. You feel your spine extending, a thick reptilian tail bursting out behind you, though it does little to impede Kaestryx's access to your hole as you instinctively lift it up and out of his way.
Your cock reshapes under you, stretching longer and wider, the fluid dripping from the tip taking on the same fiery color as Kaestryx's. Red scales continue to flow over your form as your hair falls out. Your horns curl backwards though they stop far short of Kaestryx's majestic crest. Just as your lord dragon plows into you one last time, his burning seed flooding into your gut, as a counterpoint to the fire that filled your lungs, the muscles in your back suddenly bunch up before a pair of wings burst free, stretching wide to either side. Your own orgasm overwhelms you as you exhales a stream of fire, your cock painting the ground in flame colored semen.
You master pulls free of you, growling his own satisfaction. "You truly will serve me well, stand." You do as ordered. You are a foot shorter than he is, and probably a hundred pounds lighter, but at your current size you'd still put the brawniest orc to shame. He nods his approval and steps away. Flame once again engulfs him as he returns to his full size and draconian form.
"Feel the fire inside of you, let it burn you away." You focus on those twin flames you can feel raging inside of you. And in a moment of sudden realization you let them loose. Your form burns away one more time, your body reshaping once more inside the flame, before it falls away. You stand as a dragon. Almost twice as big as you were in that draconic humanoid form, but still less than a quarter of Kaestryx's massive form.
"I have given you a gift beyond measure human. I have allowed you to ascend to dragonhood. From now on you are Halstryx son of Kaestryx. You will serve me until you are full grown and ready to start a horde of your own, in about two hundred years. Now, return to the humanoid form and acquaint yourself with my lair. I will have tasks for you later."
You instinctively pull your form in on itself as you burn away your draconic shape, and return to the seven foot tall hulk of scale and muscle your father turned you into first. You bow to your father and head off into the caverns to explore.