Oath of the Dragonrider

In terms of lifespan, most humanoids (elves being the notable exception) are to dragons what a dog or cat is to a human - a dragon that takes an interest in the humanoid races will likely live through a dozen generations of companions in its lifetime. But dragons do find a use for humanoid companions, much as humanoids do with pets, familiars and beasts. A well-trained clan of warriors or order of monks serve as excellent guards while a dragon naps for a few decades. But humanoids may be untrustworthy - stealing from the hoard while the master sleeps, or worse turning against the master. For this reason, a humanoid's oath must bind it to the dragon it serves, granting the power needed to protect the master. Thus were made the first Dragonriders.

While myth and legend may tell of humanoid heroes who tamed and rode dragons into battle, in reality, it is the dragon that tames the rider. A dragon needs no special training to fly with something less than a tenth its size upon its shoulders, merely a small amount of concern for the rider's safety, and a measure of trust if the rider sees a danger or opportunity the dragon does not. It is the rider who must learn to hang on, to avoid the horns, to ride the muscular undulations, and adjust to the buffeting of wind. Indeed, humanoid's excited faces when flying are as amusing as dogs riding on a fast chariot - utterly in awe at the marvel and grateful for the generosity and power of their masters.

You choose one specific and powerful Ancient Dragon to which you owe your allegiance. You do not get to determine when you ride the dragon. The dragon demands that you ride periodically (several years could pass between rides) for training and practice, and may take you along when it feels it could need the services of a smaller creature. You are otherwise expected to protect the lair, recover any stolen goods and severely punish thieves who would steal from your master. You are likely part of an order of Dragonriders, who might cover for you if you are on an extended mission. When the dragon dies, any surviving riders of the order undertake a quest to seek out and protect an echo of the dragon's spirit, possibly on the same plane, likely on one of the other Prime Material Planes.

The Oath of the Dragonrider is not to be taken lightly - it is not only a commitment of one's lifelong loyalty, but likely a promise of future generations of riders to be born, raised and trained to serve long after the oath has been fulfilled. More benevolent dragons may be inclined to let a Dragonrider retire in old age and live in relative peace and quiet - many would simply put down a rider too far past its prime if a suitable replacement is available. A Dragonrider who abandons his post is likely to face a far more immediate and fatal retribution than he would from a traditional oath to a deity. Even fanatical pursuit of apostates pales in comparison to a dragon's fury. 

Oath Spells

You gain oath spells at the paladin levels listed. See the Sacred Oath class feature for how oath spells work.

1           -              Alarm , Feather Fall

2          -              Levitate , See Invisibility

3           -              Glyph of Warding , Protection from Energy

4           -              Hallucinatory Terrain , Private Sanctum

5           -             Hold Monster , Wall of Stone

Channel Divinity

When you take this oath at 3rd level, you gain the following two Channel Divinity options:

Essence of the Dragon

You call upon the essence of what it is to be a dragon, giving yourself damage immunity matching that of your dragon master, and a corresponding breath weapon. The immunity lasts for one minute. The breath weapon effect is usable as an action, a number of times equal to your proficiency bonus, dealing a number of d12s equal to your proficiency bonus in either a 15' cone or 30' line. 

Visage of the Dragon

You call forth a spectral image of your dragon master, centered upon your person, granting damage resistance to your masters energy type to any creatures you choose within a 30' radius for one minute. Additionally, dragons and creatures of draconic origin recognize this as a sign of favor, and treat the Dragonrider with deference and respect. You have advantage on Charisma checks (Deception, intimidation, Performance, Persuasion) involving dragons, dragonborn, kobolds, lizardfolk, drakes, or other draconic creatures. 

Frightful Presence

Beginning at 7th Level, each creature of the Dragonrider's choice that is within 10 feet and aware of it must succeed on a Wisdom saving throw (DC 8 + proficiency bonus + Cha modifier) or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Frightful Presence for the next 24 hours.

The aura increases in size to 30' radius at 18th level.

Elemental Versatility

Conflict between different types of dragons is where the powerful Dragonrider can truly aid its master. To this end, the Dragonrider is able to absorb elemental damage and channel it offensively elsewhere. Beginning at 15th level, the Dragonrider learns and can cast the spell Absorb Elements as a first level spell at will, without expending a spell slot. The Dragonrider may choose to spend a spell-slot in order to increase the damage output, increasing the elemental damage by 1d6+1 per spell level expended.

Dragon Summoning

At 20th level you can use an action to summon a younger echo of your dragon to fight at your side. This is not your master, but a younger version of it summoned from another plane of existence. The dragon appears in an empty space within sixty feet of you and has statistics representative of it's type as listed in the Monster Manual. The dragon acts independently on its own initiative but will obey verbal commands (no action required) for the minute it is summoned. Being size large, it can be ridden by a small or medium sized Dragonrider. At the end of one minute, or if it is reduced to 0 hit points, the dragon is whisked back to its own plane.