Dragon Knight

Some ancient dragons pursue goals that inspire them to recruit agents from the ranks of mortal heroes.  Among these ancient dragons, some have mastered the innate magic of their being.  In an age past, such dragons fostered a tradition of imbuing chosen warriors with a sliver of that power, in exchange for service.  Named Dragon Knights, these warriors each possessed a bond with an ancient dragon, granting them a dragon's senses, supernatural strength and constitution, and access to their patron's innate magic.

Dragon Patron

When you choose this archetype at 3rd level, you form a mystical bond with an ancient dragon.  Talk to your DM about your dragon patron.  The color of your dragon patron will determine your damage resistance type, and its alignment may have affects on your campaign (or at least your character).  You gain proficiency in the language Draconic, and gain resistance to the damage type dealt by your dragon patron's breath weapon.  At 15th level, you gain immunity to that damage type.

Dragon's Senses

Starting at 3rd level, your senses are supernaturally heightened, granting you enhanced natural senses, darkvision, and the ability to sense vibrations in close proximity.  You gain proficiency in the perception skill if you don't already have it, expertise in the perception skill, darkvision with a range of 120 feet, and blindsight with a range of 10 feet.  At 18th level, the range of your blindsight increases to 30 feet.

Dragon's Long Life

Starting when you choose this archetype at 3rd level, you are granted the lifespan of a dragon as part of your bond.  You age at a geologic pace.

Innate Spellcasting

At 7th level, you gain the ability to tap into your patron's magic.

Innate spellcasting.  You learn a number of spells equal to your charisma modifier, with a minimum of 1 spell.  These spells can come from any spell list.  Each time your charisma modifier increases (except for when it increases to a value of +1 or lower), you learn a new spell.  All spells learned from this feature must be 1st or 2nd level, and are always cast at 2nd level if applicable.  Each spell can be cast once per day, requiring no material components.

Spellcasting Ability.  Your spellcasting ability is Charisma for your Innate Spellcasting.  You use Charisma whenever a spell refers to your spellcasting ability.  In addition, you use your Charisma modifier when setting the saving throw DC for an innate spell you cast and when making an attack roll with one.

Spell save DC = 8 + your proficiency bonus + your charisma modifier

Spell attack modifier = your proficiency bonus + your charisma modifier

Dragon's Constitution

Starting at 10th level, your contact with the magic of an ancient dragon supernaturally boosts your fortitude.  Increase your constitution score by 2.  The maximum your constitution score can be increased to is now 22.  At 15th level, it can be increased to a maximum of 24.

Improved Innate Spellcasting

Starting at 15th level, you are granted increased access to your patron's magic.  You gain the ability to manifest a more powerful magical effect.  Replace 1 known spell from your Innate Spellcasting feature with a spell of 3rd level.  At 18th level, replace 1 Innate Spellcasting spell with a spell of 4th level.

Dragon's Strength

Starting at 15th level, your extended contact with the magic of an ancient dragon supernaturally boosts your strength.  Increase your strength score by 2.  Your strength score can now be increased to a maximum of 24.

Dragon's Spirit

At 18th level, you are a valued asset to your patron, and they will step in to protect you in a moment of need.  When you fall below 1 hp, you can use your reaction to be instead reduced to 1 hp and gain 50 temporary hit points, which last until you take a long rest.  You become possessed by your dragon patron, and the DM will temporarily take control of your character, acting as the ancient dragon possessing your character's body.  While in this state, you have a blindsight range 60 feet, and your Intelligence, Wisdom, Charisma, Strength, Dexterity and Constitution scores are replaced by your patron's.  The possession lasts until the temporary hit points run out.  The ancient dragon can choose to end the temporary hit points and return control to the character as an action.  When the state of possession ends, you gain 2 levels of exhaustion, and can't use this ability again for 2d4 days.

Unleash Breath Weapon (Optional)

This 18th-level feature replaces the Dragon's Spirit feature.

As an action you can unleash the power of your patron's breath weapon, exhaling devastating energy.  Your breath weapon has the same damage type and target saving throw ability as your patron's.  If your patron's damage-dealing breath weapon is a cone, your breath weapon takes the shape of a 90-foot cone.  If your patron's damage-dealing breath weapon is a line, your breath weapon takes the shape of a 120-foot line that is 10 feet wide.  Each creature in the the breath weapon's area must make a saving throw with a DC equal to 8 + your Constitution modifier + your proficiency bonus, taking 13d10 damage on a failed save, or half as much damage on a successful one.

Unleashing this power takes a toll on your body, and after doing so you are incapacitated until the end of your next turn and take 2 levels of exhaustion, which can only be removed by completing rests.