Mysterious Origin

You remember very little of your first years.  You don't remember a mother, or a father, or any other family.  You do remember the wilds, and years of making your way by scavenging, foraging, and occasionally trading in small towns, or to travellers.  Always quick to make friends, particularly where it suited you, but nearly as quick to leave them behind.  You felt impelled to always be moving on, though you know not why.

Throughout your childhood you felt an feel an immense, intangible pressure building inside you - like a massive dam steadily filling with water.  You had no idea what it was, but you knew it was immensely powerful.  When you reached maturity the dam felt full to bursting.  You started feeling leaks.  While seemingly tiny they manifested as powerful abilities.  That was how you discovered you have innate magic.

You know as little about your destiny as your origin, although you find you know things you've never learnt - things about arcana, and magic, and people's true intentions.  You also find that you are broadly familiar with places all over the world to which you've never been.  You speak several languages natively yet you can't explain how.  But what truly shocked you was when you first encountered a dragon.

You witnessed a young red dragon raze a village.  The attack was shocking enough, but what truly sent goosebumps over your soul was when you realised you could actually understand the dragon as it jeered and mocked the dying.  Nobody else seemed able to understand its roars and guttural sounds.

You felt powerfully moved to aid the villagers, far more than anyone could explain given how barely you knew them.  Alas, you could do nothing useful against a dragon.  You yourself barely survived.  It was that attack which led you to the adventuring life, as you felt compelled and resolved to find a way to put an end to the cruel, evil dragons.

Instructions (spoilers for players!)

Note: D&D Beyond screws up the formatting of these instructions when viewed in the Character Builder.  View via the Homebrew background browser instead.

Important:

• If you are a player, you must secretly work with your DM in advance to ensure they approve this background's use and are willing to craft it into the campaign appropriately.  As the player you should be respectful of this background's significant roleplaying requirements and must be up to the task.
• If you are a DM, you could choose this background and not reveal the subterfuge to the player in advance, so they are just as in the dark about their character's true nature as the rest of the players.  This works well if you are providing pre-made characters.
  
  If you wish to impose it involuntarily on a player-invented character, think carefully about how that player will react.  While this background ultimately provides great power and abilities to the character, it may conflict with the player's own goals and conception of their character.

Prerequisites

Race: A humanoid race common to the game world and starting area.
Class: Sorcerer.

Summary (spoilers!)

You are a dragon that was manipulated with incredibly powerful magics as a youngling, by which you were transformed into a very plain humanoid, with your memory and your draconic attributes & abilities suppressed.    You lived a relatively mundane life, unaware of your true nature, until you became an adventurer - the exhilaration and stress of the adventurer's life awakens your true nature, causing you to slowly revert to your true form, and revealing your destiny.

Either work with the DM on a secret destiny revolving around your dragon ancestry - which you'll need to role-play that you're ignorant of; that your character has to piece it together over time in response to seemingly random hints and events - or have the DM come up with something privately, so even you don't know your true origin story nor destiny until they're revealed in gameplay.  Example:

Your parents needed to protect you from an evil so powerful that even they, as Ancient Dragons, couldn't defeat it.  So they faked your death and orphaned you under a disguise so effective even you don't know your true nature and identity.  As your true form is revealed, the Great Evil from which your parents hid you becomes aware of your continued existence and begins hunting you down.

Phase 1

You appear as a humanoid of your choice to begin with - your facade race - with all its normal starting attributes and abilities.  Your chosen race must be common and unremarkable in the context of your game world and starting location.

Your size and visual characteristics are exceptionally average for your race - as if your whole appearance were designed to be as unremarkable and forgettable as possible (though as the character you do not know this, and be careful not to draw any attention to your averageness when you introduce the character to your fellow players).  You have no tattoos, piercings, or similar identifying visual characteristics unless those are strictly necessary to your race (e.g. as a human you have no tattoos or piercings, but as a giant you would have the minimal set of tattoos expected for clan identity).  You have no scars or deformities.

Phase 2

At suitable story moments - chosen by the DM - you unwittingly discover that you have Dragonborn abilities.  e.g. Darkvision (if your facade race didn't have it already), Damage Resistance, and Breath Weapon.  Typically these manifest at separate times.

Once the first such ability is revealed by a suitably significant event, begin slowly revealing further abilities, intermingled with appearance changes, e.g.:

1. Your skin starts to subtly tint towards your inherent [dragon] colour.
2. Your skin becomes harder, dryer, and eventually scaly.  You now have the 'Grotesque' attribute:
   • -1 to all charisma-based rolls other than spellcasting, as a result of your disturbing and unusual appearance.  This can be neutralised if you use a physical disguise or covering to hide your appearance.
3. An attack (cold / fire / etc as appropriate to your dragon type) hurts notably less than you expected (Damage Resistance manifesting).
4. Your body slowly develops to be more refined and ideal to your gender.
5. You realise that you can still see in dim light, where you would not have been able to previously (Darkvision manifesting)
6. You start growing actual horns, your finger- and toe-nails enlarge and extend into claws, your facial features become harder and more draconic.  Your 'Grotesque' attribute increases in strength:  now -2.
7. If you're not already as big & muscular as a large Dragonborn (7', 350lb), you slowly become so.
   • Be mindful of the effects of size changes, e.g. any clothing or armour you wear might no longer fit.  Initially (if using incremental changes) this will manifest as losing proficiency with the item, but inevitably it may mean the item cannot be worn at all.  It may be possible to tailor, leatherwork, or blacksmith the items to fit again (though this may have to be performed repeatedly, as the growth continues).
8. You unwittingly make a Breath Weapon attack (either in combat as a substitute for a normal attack but under exceptional circumstances, or unwittingly at an [in]opportune story moment).

During or at the culmination of your transformation - at the DM's discretion - your character stats change appropriately:

• Your attributes adjust from your nominal starting race to Dragonborn (Str + 2, Cha + 1).  You lose the attribute benefits of your facade race, e.g. the extra dexterity of an elf, the constitution of a dwarf, etc.
  • If your DM is particularly charitable they might omit some or all stat reductions that are technically required, such as if it makes sense to do so within your character's unique, personal story.
• You lose physical abilities granted by your facade race that aren't also intrinsic to Dragonborns, unless they make logical or story sense to keep.  e.g. the poison resistance of a dwarf.
  • You keep proficiencies in weapons, tools, languages, musical instruments, and other knowledge-based skills (e.g. Stonecunning), since those are all learned abilities from your upbringing.

The visual aspects of transformation can - at the DM's discretion and direction - be revealed either:

• Continuously with subtlety.  e.g. occasionally the DM could have the party make a perception check, and reveal a change in the player's appearance on success.  e.g. "You realise, as you look at <Character>, that their skin glints in the light on certain angles, as if ever so slightly scaly".  Tie this into gameplay events as best you can, e.g. when one or other characters have reason to be unusually close or attentive to the character in question.  For extra fun try to time it along with the character being poisoned or affected by mysterious magic, so that the players might think it's a curse or similar externally-driven change.
• Marked by aperiodic, abrupt changes.  Potentially harder to work naturally into the gameplay events.  e.g. if the character makes an unarmed attack, the DM could say "You pull your hand back in a fist, ready to punch, but desperation and feral instinct drive your movements, and inexplicably your fist opens to reveal vicious claws, which you rake across your opponent to horrific effect…".

It's wise to not announce changes to the other players, but rather let it come out naturally through gameplay (e.g. do they actually notice that your modifiers and saving throws change?).  If they do notice, act as confused as they are.  Gaslight them, or blame the DM if you have to, but either way preserve the mystery as much as possible.

Eventually you appear as fully and natively Dragonborn.  You lose the Grotesque attribute, but are still subject to all the usual prejudices of being a Dragonborn.

During this second phase it can be helpful (for gameplay and/or player jealousy) to balance the positive improvements with some transient negative ones (such as the 'Grotesque' attribute in the example).  This can include temporary stat reductions - particularly to dexterity and charisma - as a result of the confusing and dramatic changes to your physicality.  The DM can decide if and what these are, to best suit the campaign.

Phase 3

After playing as Dragonborn for a while - and perhaps even playing through a significant story arc revolving around the notion that you are truly a Dragonborn and this is the entirety or your secret - find some suitable campaign event to trigger the next transformation.  e.g. you unwittingly cross paths with the Great Evil that threatened you as a baby.  The DM will need to arrange for this to be only a prelude, with the Great Evil and the party getting away from each other relatively unscathed, but thus laying the groundwork for the climax of your transformation story.

Once triggered, this second transformation follows the same general pattern as the transformation to Dragonborn in Phase 2, except now it's from Dragonborn to Ancient Dragon.  Directed by the DM, it can proceed as e.g.:

1. Your constitution increases by 2.
2. You realise that you no longer need to rest in-between uses of your Breath Weapon (but cannot use it more than once per minute).
3. Your intelligence increases by 2.
4. You discover you can sense nearby creatures even when you can't see them (Blindsight manifesting).
5. Your constitution increases by 2.
6. Attacks using your dragon nature no longer harm you at all (Immunity manifesting, from mere Resistance previously).
7. Your charisma increases by 2.
8. You realise that one or more spells that involve your dragon element (e.g. fire for Gold dragons) have become so intuitive for you that they do not count against your spell limit.
   • If you'd already learnt them, you may select replacement spell(s) either immediately or at a story-suitable later time - DM decides.
   • If you have not learnt any suitable spells, but there are such spells available to you (per normal Sorcerer requirements, feats, magic items, etc), then you spontaneously acquire one or more of them - DM decides.
   • Typically, if there are multiple eligible spells, this should occur one by one over time, starting with the lowest level spell.
9. Your dexterity increases by 2.
10. You discover that you can breath underwater (Amphibious manifesting).
    • This can be fun for the DM to reveal through a bit of deus ex machina - e.g. by putting you in a situation where you unavoidably start drowning, at which point the DM can say "You've been holding your breath, but your lungs burn excruciatingly and the impulse to breath becomes over-powering.  Despite your best efforts, you cannot stop yourself gulping for air - but of course you instead get a lung full of water.  Yet you feel a sudden relief as oxygen surges into your arteries once more, and you realise you can, apparently, now breath under water.".
11. Your strength increases by 2.
12. If not already Large (most humanoid races are Medium, or Small) you increase in size to Large, with appropriate changes in height, weight, etc.
    • If appropriate, the DM can mete out the size increase in smaller increments, especially if it helps downplay the size change.
    • Any clothing or armour you wear will no longer fit.  Initially (if using incremental changes) this will manifest as losing proficiency with the item, but inevitably it means the item cannot be worn at all.  It may be possible to tailor, leatherwork, or blacksmith the items to fit again (though this may have to be performed repeatedly, as the growth continues).
    • You gain the 'Frightful' attribute:
      • -5 to Persuasion, Performance, and charisma-based saving throws along those lines.
      • +5 to Intimidation.
      • You gain the Intimidating Presence action of the Barbarian Path of the Berserker.
    • You gain the 'Awkward Growth Spurt' attribute:
      • -6 to your dexterity.
      • You lose a level in any proficiencies you had in Athletics, Acrobatics, Sleight of Hand, and Stealth.  i.e.:
        • if you were half-proficient, you are no longer proficient at all.
        • If you were proficient, you are now only half-proficient.
        • If you had expertise, you are now merely proficient.
13. A tail grows from your lower back, granting you a new melee weapon attack (DM to determine hit modifier and damage as appropriate for the character's level at the time of this development).
    
    • 'Frightful' is replaced by the 'Terrifying' attribute:
      • -10 to Persuasion, Performance, and charisma-based saving throws along those lines.
      • +10 to intimidation.
      • Good creatures will be inherently fearful of or hostile towards you.  This doesn't necessarily mean they attack you spontaneously - depending on their nature and strength relative to you / your party, they might simply hide, try to escape your presence, refuse to talk or deal with you, try to gather a mob to run you out of town, etc.  Larger settlements may refuse you entry or call the city guard against you.  If you also behave in any way dangerous or evil towards the settlement, they may hire mercenaries or other bands of heroes to hunt you even outside their borders (more readily and vindictively than they would have otherwise).
      • During combat or whenever you are otherwise acting violently, Intimidating Presence automatically (and unwillingly) invokes against every creature of your size class or smaller within 30 feet of you.  It does not cost you an action, and occurs immediately when the creature comes to be within 30 feet of you.  This includes party members, allies, and creatures otherwise not hostile to you, unless they have known you closely and continuously throughout Phase 3.
        • It does not automatically extend past your next turn, but you may use your action to extend its effects.
14. You lose the 'Awkward Growth Spurt' attribute, as you finally adjust to your new form.
15. You regain some or all the proficiencies you lost earlier.
16. Wings sprout from your back, granting you the ability to fly (initially at half your walking speed, improving incrementally over time to 120 ft).
17. You gain proficiency (if you don't already have it) in Acrobatics and Athletics.

The sequence above tries to front-load changes which could be explained as you simply becoming more powerful (a "goosed" Dragonborn), leaving the more revealing physical changes to the later stages.  The DM is free to use their own order and process as befits their campaign and your unique circumstances.

Attribute changes can be coupled with attribute changes or separated out into leading indicators, at the DM's discretion.  Attribute changes may also be scaled up or down, and reduced or increased in frequency, at the DM's discretion (e.g. to account for the party's current level).

Endgame

The final transformation occurs during the 'boss fight' with the Great Evil.  At a suitably dramatic moment - e.g. when you would normally be knocked unconscious or killed - the entire concealment spell over you finally shatters, violently.  e.g.:

A massive wave of energy explodes outwards from you.  Every creature within 120 feet is knocked prone.  Creatures within 60 feet are pushed directly away to 60 feet (and may suffer force damage at the DM's discretion).  The explosion creates a huge cloud of smoke, dust, and debris, which dissipates to reveal a fully-grown Ancient Dragon!  You now have the stats and abilities of the Ancient Dragon (of the appropriate colour).  In dragon form you retain all your equipment.  You also still have all your knowledge, skills, spells, and abilities where it makes sense, but you might not be able to use them while in dragon form (e.g. you cannot use most sets of tools, or most weapons & wearables for that matter, because they are simply too big).  You have full health and all harmful combat effects (e.g. blindness, paralysis, etc) are removed.

Important:  this miraculous unconsciousness / death save cannot happen if you have not first progressed through the transformation as previously described.  The magics that concealed you were no meagre illusion or transmutation, but ancient, powerful magic that truly changed your being.  You must slowly breach the deep magics, weakening them over a long period, before they can possibly be broken.  So if you are knocked unconscious earlier in your story, you fall unconscious like normal.  Likewise if you are killed, you die.

You lose the 'Immutable form' attribute.  Any detrimental effects resulting from your gradual transformation are dismissed (e.g. any lingering proficiency loses, negative ability modifiers, etc).  You also regain all your childhood memories, including memory of the Great Evil and how it attacked your family.  The DM might use this opportunity to provide hints about how to defeat the Great Evil, if necessary.

You gain the War Caster feat, if you don't already have it - dragons are natural quadrupeds, so somatic elements to spell-casting would just be annoying.  You have access to all your spells while in dragon form, as well as any necessary material components in your inventory (your giant clawed hands aren't great for dealing with itty bitty component pouches, but your ancient, intrinsic magic can make use of components no matter where they are on your person).

You may use the Change Shape action to - in addition to its normal behaviour - return to your prior forms (with suitable tweaking by the DM of character stats, to properly reflect your current level).  You may choose the precise degree of the form from any you've had before (e.g. pure facade race, pure Dragonborn, oversized Dragonborn, Dragonborn with wings, etc) but may not arbitrarily mix-and-match attributes.  If there is uncertainty about what your attributes were at a particular version of your past form, the DM decides.

Aftermath

This is where it might get difficult for you as the player:  you may need to give up this character.  A full-blown Ancient Dragon - plus all the benefits of a lifetime of adventuring as a Sorcerer - is likely far too imbalanced for your party.  It is likely necessary to fashion an end to your story that removes you from play.  Remember to keep the meta-gaming to a minimum - maybe you gift each of your companions one of your treasured items each, as a parting gift, but really you need most of your stuff because your life will go on elsewhere.  Plus, you are a dragon after all - you instinctively do not want to give up your riches.

If your DM does allow you to play on:

You have now discovered your true form and identity, which closes a major chapter of your life.  But there is plenty more adventuring to be had.  Onwards!

No more origin-driven changes apply, though at the DM's discretion your personal story may continue, e.g. once the Great Evil is defeated, and now that you remember your childhood and your family, you might set out to find your surviving relatives (if any).

Skill Proficiencies: Having grown up largely alone in the wilderness, you are proficient in Survival.  You are also proficient in Arcana, History, and Insight - although those you can't explain.

Languages: You speak the most common language(s) of your race, of course, but you also know Draconic - though you don't know how.

Languages seem to come easily to you, so you speak two additional languages of your choice (prefer regular languages that make sense in context for someone growing up in the wilderness).

Suggested Characteristics

d8	Personality Trait
1	I grew up alone and it's comfortable to me. I'm hesitant to form lasting relationships.
2	I miss my family immensely, even though I don't remember them. I'm particularly sensitive to broken families.
3	As a child I was flighty and shy, but I'm starting to grow out of it.
4	I don't have much experience around people, but they've proven quite malleable and I'm used to easily getting what I want.
5	Growing up alone in the wilds was not easy. I'm hardy, independent, and self-confident.
6	I don't talk about my childhood, or my past in general. I live in the here and now.
7	I hate dragons with a fiery passion. Yet I am secretly fascinated by them.
8	I have a wisdom that's far beyond my years.

d6	Ideal
1	I will hunt down and destroy every last dragon.
2	I will always help those in need - the more dire their need, the stronger my conviction.
3	Nothing is more important to me than discovering my origins. I will do whatever it takes.
4	There is only me. I was seemingly born alone and I will die alone. Everyone else is less.
5	Something terrible happened to me as a child, I just know it. I will have vengeance, against the world in general.
6	I've been alone all my life. I know how it feels. I will never be alone again - friends are everything to me.

d6	Bond
1	I was raised by hunters, outlaws, and nomads. They are my people.
2	I grew up in and survived on the natural world. I am forever grateful and connected to the land and the animals.
3	I feel connected to my parents. Even though I don't remember them, my imagination of them keeps me going.
4	Something terrible happened to me - that must be why I don't remember. I fear I'm dangerous. Am I evil?
5	I just know that my soulmate is out there. Not hypothetically or figuratively, literally and actually. I will find them, and they will be everything to me.
6	I never had people in my life, not really, but this magic has been with me all along. It is part of me and makes me part of something greater.

d6	Flaw
1	I should have saved that village from the dragon. Should have fought. I will never again back away from a fight.
2	I am a loner to a fault. I work with others only when and to the extent it benefits me.
3	Growing up in the wilderness, alone, means missing out on a lot of things - like… romantic experience. I am rendered an incompetent fool by anyone I'm sexually attracted to.
4	I never had a family, so I lack empathy for family relationships. The way noble families work baffles me.
5	I've never had a meaningful relationship with another person, so I've never learnt to trust. I will never rely on someone else for anything important.
6	Everything comes easy to me - what's wrong with everyone else? I have no sympathy for ignorance or lack of skill.