Dragonborn are Irradiated Humans Who Experience Metamorphosis From A Violent & War-Filled Life

When a dragon (of any stripe; some are elementals, some are greedy dwarves, and some are extinctions made flesh) dies, its corpse bleeds literal majesty into the world around it. Think of this majesty as something akin to radiation. It seeps into the soil, spreads on the wind, rains from storms.

Some years later, children are born. Children with ill temperaments, obsessed with fire or lightning or something just as dangerous. These children, male and female, are not evil, but they are violent. They fight. They push. They are always human at first.

Sometime in their adult years, they will be hurt. Their blood will be of a strange color then, not the red it was of their youth. That strange color will harden over their flesh. Scales emerge, in small sections. A mercenary whose eye was hurt is now slitted, and half his face covered in blue scales, and from his spine horns have burst through the skin, spreading scales more. Like finds like, more often than not. Sellswords, all in various stages of rebirth, mixtures of scale and human, make for fantastic soldiers.

A war wizard mid transformation.

Eventually, after enough pain has been suffered, nothing human is leftover. Know this as a Dragonborn in true. Something once human that has given into the majestic violence that has gathered in its soul. Capable of breathing the same magic as a dragon, a warrior through and through. Not evil, not always, and not always good either. Warrior poets and paladins and barbarians across the world--entire battlefields of fielded quasi-dragon and full-fledged dragonborn mercenaries fielded and thrown at once another.

Full-fledged dragonborn

Their society is an odd one. Full-fledged dragonborn gather in clans and hide away from the rest of the world, usually at least. They will emerge only during times of war, pledging their swords to one side or the other, collecting gold, hoarding gold. They are draconic, after all. They will search for those just beginning their transformation, or others deep into it. They will guide them into more battles, seduce them into their warbands, nurture them into true dragonborn. They cannot reproduce, after all, irradiated with majesty as they are. Their clans grow by recruitment.

Dragonslayer experience more change.

Sometimes they do choose to live amongst other peoples. Rare this is, but they will use their veteran's prestige to fund their manses and workshops. These dragonborn, sometimes half-formed, other times fully formed, must work hard to control their violent impulses. It is difficult. Usually, it is not worthwhile.

Others still seek more dragons, and become famed dragonslayers. Their instinct pushes them to do this. Their souls devour majesty. Soon, they become half-dragons and, if they are lucky, dragons in full too.

Half-Dragon Well on the way to becoming a dragon in true

All manners of draconic creatures (Wyverns, Fire Drakes, Ice Wurms, etc) are born in much the same manner--mundane animals born with majesty that, through a violent life, become something new.

Have an OD&D class for this style of Dragonborn.

Dragonborn

Dragonborn may progress as a fighting-man up to 8th level (Champion). They may use magical weapons, but enchanted items fail in their hands, and magical armors cannot be donned. When a dragonborn first suffers 6 points of damage, roll a 1d12 on the table below. The dragonborn evolves thusly. After the dragonborn suffers twice that damage (12), it rolls again. And again after twice that (24), and so on and so forth. If the dragonborn rolls a feature it already has, it does not reroll, but simply remains stable. Damage suffered does not need to be at all at once, and can be suffered in pieces in between healing of any kind. A dragonborn who gains all 12 features retires at level 8, going into hibernation and reawakening as an 8 HD dragon wyrmling.

Draconic Features

1. Scales appear where your wounds were. Lower your AC by 1 (or increase by 1 if using Ascending AC).
2. Magic can barely hold purchase in your soul. You save vs magical effects as if you were 4 levels higher.
3. Your eyes turn the color of gold coins, your pupils into slit-daggers. You have infravision in darkness, and can see golden coins through walls no more than 1 inch thick.
4. Your musculature changes. Increase your Strength modifier by 1.
5. Your flesh becomes enriched with power. Increase your hit points by 1d6.
6. Claws burst from your hands and feet, and your legs become bow-legged. You can no longer ride non-draconic mounts, and your hands and feet count as off-hand daggers.
7. Wings burst from your upper back. Armor must be specifically modified to accommodate this. You can move 3" on your turn through flight.
8. A tail bursts from your lower back. Armor must be specifically modified to accommodate this. Your tail counts as a limb for moving, holding, or pushing objects, but not weapons or shields.
9. A set of horns burst from your head. Helms must be specifically modified to accommodate this. They count as daggers.
10. Your mouth becomes a hellish hole of sharpened fangs. Your bites count as off-hand dagger attacks.
11. Your voice becomes monstrous, draconic. Hirelings do not fail morale checks in your demanding presence, and creatures subtract 5 from their morale scores against you.
12. Something apocalyptic dwells in your breast. You gain the breath weapon of a dragon wyrmling.

Decide your own scale color/type, but stick to it.