Dragon Knight

Not many deserve to rule over dragons, and fewer still can ever accomplish such a feat, but those who do mostly become Dragon Knights, an ancient order who once devoted their lives to protecting the eggs of dragons. These warriors would lay down their lives in exchange for the dragon to watch over their race. Occasionally a wyrmling would see the devotion of these knights and would then devote their lives to protecting the one who they owed their chance at life to, but with the dwindling numbers of the dragons, this order simply faded into history. Occasionally a mortal might become a Dragon Knight by raising a dragon, although many have found ways to enslave dragons to their own ends. Neither good nor evil prevents a Dragon Knight from taking form, all it takes is devotion.

Dragon Companion

Starting at 1st level, you gain a dragon companion. Your dragon ages magically as you grow in power and size as you level in this class. When you gain this feature choose a dragon type based on your alignment, at least 1 axis of which you must share with the dragon. You also learn Draconic; if you already know Draconic, you may instead teach a language you know to your dragon. You also gain the feat Mounted Combatant with your dragon, but only if it is large enough for you to ride. You also form a telepathic bond with your dragon (like that in the find familiar spell). Dragon Knights are known to wear armor in the fashion and color of their dragon.

The DC against your dragon's abilities is equal to 8 + your dragon's proficiency bonus + your dragon's Charisma modifier. The saving throw needed for opposing the DC is listed next to the breath weapon.

Dragon Template

Dragon Template
Your dragon levels with you, as though it had a class, it gains 1d12 + its Constitution modifier in hit points each time you level, it starts with 12 + it's Constitution modifier hit points. Your dragon cannot wield weapons, it is considered proficient with its own attacks, it uses 8 + its proficiency bonus + its charisma modifier for its DCs. Its AC = 13 + Dexterity Modifier unless wearing armor specifically tailored to the dragon's size and shape, which due to its specialized nature, will cost 4 times as much as the base armor. In which case, the dragon has the AC provided by the armor. If your dragon dies (same rules as a player death) you are unable to regain a different dragon, but instead must resurrect or reincarnate your original dragon, either through magic or a quest given by the DM. Your dragon is proficient with 2 skills of your choice and is proficient in one saving throw between Dexterity and Constitution. The dragon has a walking speed of 30 feet, and immunity to its element. Your dragon improves as you level, as shown on the table below. The dragon has the following ability score restrictions: Tiny, Medium, and smaller: Max of 24, Large: 26, Huge: 28, Gargantuan: 30 and it can't trade ability score increases for feats (it gains feats a different way).

Dragon Knight Level: Size: Proficiency: Bonus Attacks: AC: Bonus Features
1 Tiny    +2 None 0 Major Trait, Minor Trait
2 small    +2 Bite: 1d4+Str 0 Ability Score Increase, Minor Trait
3 Small +2 Bite: 1d4+Str 1 Major Trait
4 Small +2 Bite: 1d4+Str 1 Ability Score Increase, Minor Trait
5 Small +3 Bite: 1d4+Str 1 Major Trait
6 Med +3 Bite: 1d6+Str 1 Ability Score Increase, Minor Trait
7 Med +3 Bite: 1d6+Str 1 Major Trait
8 Med +3 Bite: 1d6+Str 2 Ability Score Increase, Minor Trait
9 Med +4 Bite: 1d6+Str 2 Major Trait
10 Med +4 Bite: 1d8+Str, Claw: 1d4+Str/Dex 2 Ability Score Increase, Minor Trait
11 Med +4 Bite: 1d8+Str, Claw: 1d4+Str/Dex 2 Major Trait
12 Large +4 Bite: 1d8+Str, Claw: 1d4+Str/Dex 2 Ability Score Increase, Minor Trait
13 Large +5 Bite: 1d8+Str, Claw: 1d4+Str/Dex 3 Major Trait
14 Large +5 Bite: 1d10+Str, Claw: 1d6+Str/Dex, Tail: 1d4+Str 3 Ability Score Increase, Minor Trait
15 Large +5 Bite: 1d10+Str, Claw: 1d6+Str/Dex, Tail: 1d4+Str 3 Major Trait
16 Large +5 Bite: 1d10+Str, Claw: 1d6+Str/Dex, Tail: 1d4+Str 3 Ability Score Increase, Minor Trait
17 Huge +6 Bite: 1d10+Str, Claw: 1d6+Str/Dex, Tail: 1d4+Str 3 Major Trait
18 Huge +6 Bite: 1d12+Str, Claw: 1d8+Str/Dex, Tail: 1d6+Str 3 Ability Score Increase, Minor Trait
19 Huge +6 Bite: 1d12+Str, Claw: 1d8+Str/Dex, Tail: 1d6+Str 3 Major Trait
20 Garg. +6 Bite: 1d12+Str, Claw: 1d8+Str/Dex, Tail: 1d6+Str 5 Ability Score Increase, Minor Trait
Attacks
The bite attack deals piercing damage and has a reach of 5 feet, this range increases to 10 feet at 7th level.
The claw attack deals slashing damage and has a reach of 5 feet, this range increases to 10 feet at 13th level. You can use your Strength or Dexterity modifier for your attack and damage rolls with this attack.
The tail attack sweeps around the dragon in a circle. All creatures in a 10 foot radius must make a DC 8 + the dragon's proficiency bonus + the dragon's Strength modifier Dexterity saving throw save or take the bludgeoning damage listed.

Minor Traits
When your dragon gains access to a minor trait it may choose one of the following.

Echoing Voice: The dragon's voice bellows out and can be heard up to a mile away, must be at least large sized.
Proficient: Your dragon may choose 1 skill of your choice to gain proficiency in, you may select this trait multiple times
Language: Your dragon learn a new language, you may select this trait multiple times.
Sight: The first time your dragon chooses this it gains dark-vision out to 60 feet, the second time they gain blind-sight out to 10 feet.
Amphibious: Your dragon can breathe in water.
Ice Walk: The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally difficult terrain composed of ice or snow doesn't cost extra movement.
Quick: Your dragon's walking speed increases by 10 feet, you may select this trait multiple times.
Swimmer: Your dragon gains a swim speed equal to walking speed.
Flight: Your dragon gain the ability to fly at a speed equal to twice its walking speed.
Burrow: Your dragon gains a burrow speed equal to its walking speed.
Immunity: Choose charmed, exhaustion, frightened, or paralyzed, your dragon gains immunity to the chosen condition. You may select this trait multiple times.

Major Traits
When your dragon gains access to a major trait it may choose one of the following.

Feat: Your dragon gains a feat of that it qualifies for, you may select this trait multiple times selecting a different Feat each time it does.
Saves: Your dragon gains proficiency in 1 of the following saves: Intelligence or Charisma.
Caster: You and your dragon learn one spell upon this trait's selection from the sorcerer spell list. These spells are recharged after a long rest. Your dragon must be small or larger to select this feat. Every size category increase gains access to the next level spell. Tiny 0 level, small 1st level, etc. You and your dragon use charisma as the spell-casting modifier. If either of you use the spell the other can't use it also until a long rest. You may select this trait multiple times.
Change Shape: The dragon magically polymorphs into a humanoid or beast that has a CR no higher than 1/4 its knight's level, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice). In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, resistances, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.
Hypnotic Speech: Your dragon may speak to a non-hostile creature, the target makes a wisdom saving throw against (8 + proficiency + your dragon's Charisma bonus). If they fail they become charmed by your dragon for 1 day, then become immune to this effect for 24 hours after it ends. If a creature succeeds their saving throw they do not get charmed and become immune for 24 hours. Your dragon must be at least small size to select this trait. The dragon has one use of this ability, and regains the use of this ability after the completion of a long rest

Breath Weapon:

Beginning at level 3, as an action, you may command your dragon to use its breath attack. If your dragon has an intelligence of 12, it may perform its breath attack on its own. After having used breath weapon, at the start of each of your turns, roll 1d8 -- on a 8 it can use breath weapon again. The change of recharging gets better as you level, as shown in the Damage/Recharge Requirement column of the table below. The breath is either a cone or a line when using a breath attack depending on the dragons color. If your dragon already has a preexisting breath attack, these bonuses do not stack. The old breath attack is removed and the new one is put in its place.

Dragon Size Damage/Recharge Requirement Area
Tiny 2d6/8 Cone:10/Line:20
Small 4d6/7-8 Cone:10/Line:20
Medium 6d6/6-8 Cone:20/Line:40
Large 8d6/6-8 Cone:30/Line:60
Huge 10d6/5-8 Cone:40/Line:80
Special Breath Attacks
Some dragons have specialized breath attacks, these attacks do not deal damage, but cause different effects, these effects only take hold on a failed save. The specifics for time spent under a condition, or how far an enemy moves from Repulsion, is listed in the table. Your Dragon knows one and can use it once per long rest.

Sleep: Causes a creature to fall unconscious, this effect ends if the creature takes damage or a creature uses an action to wake it up.
Repulsion: Pushes the target from the dragon.
Slowing: The creature can't use reactions, its speed is halved and can't make more than one attack on its turn. In addition, the creature can use either an action or a bonus action on its turn, but not both. The creature can repeat the saving throw at the end of each of its turn, ending the effect on itself with a successful save.
Weakening: Creature has disadvantage on Strength based attack rolls, Strength checks, and Strength saving throws. The creature can repeat the save at the end of its turns, ending the effect on itself on a successful save.
Paralyzing: Causes the creature to become paralyzed . A creature can repeat the saving throw at the end of each of its turns, ending the effect on a successful save.

Master of the Saddle

At level 3, You have advantage on saving throws made to avoid falling off your mount. Mounting or dismounting a creature costs you only 5 feet of movement, rather than half your speed. Additionally all attacks made while mounted have advantage.

Calm Under Pressure

Starting at 7th level, You can move up to twice your movement speed for the purposes of getting to your dragon, Additionally you will not provoke opportunity attacks while running to your mount and getting saddled.

Twin attackers

Starting at 10th level, should an enemy attack you, your dragon can make an opportunity attack against your attacker. Should an enemy attack your dragon you can make an opportunity attack against your dragons attacker. On a successful hit your enemies movement speed is reduced to half and must find a new target. 

Momentous force

At 15th level, you can run down your foes, whether you're mounted or not. If you move at least 10 feet in a straight line right before attacking a creature and you hit it with the attack, that target must succeed on a Strength saving throw (DC 8 + your proficiency bonus + your Strength modifier) or be knocked prone. You can use this feature only once on each of your turns. If you are mounted you can add one damage die and the strength modifier of your dragon to the DC or damage roll.

Destructive Strikes

At level 18 when attacking from your mount 18+ on the D20 is counted as a critical. Your critical will add an additional damage dice, Your dragon gets a critical attack as well and your dragons breath weapon is recharged. On your critical you can also add an effect such as stunned, knocked prone, disarmed, blinded, deafened or feared.

The effect ends at the end of the creatures next turn.

Additionally once per turn, you can add your dragons elemental type onto your strike in the form of 3d6.