Xar'Athur have a +1 bonus up to three times to boost any ability they desire.

You are immune to fire, poison and sleep. You take no damage from fire and poison attacks thanks to your demonic heritage. Sleep effects (spells, abilities, potions etc) do not affect you thanks to your dragonic heritage.

The demonic blood you posses gives you the ability to resist cold. Your resistance to cold lets you ignore the first 5 damage you may suffer from cold attacks each turn.

You can telepathically communicate with any creature that posses a language within 60 ft of you. This ability was also bestowed to you from your demonic heritage.  

The Xar'Athur can make use of its Frightful Presence to makes three attacks: one with its bite, one with its claws and one with its tail.

You make use of your sharp teeth to attack your enemies. Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 2d4 piercing damage.

You make use of your dragon claws to attack your enemies. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 2d4 slashing damage.

You use your dragonic tail to attack your enemies. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit:  1d6 bludgeoning damage.

A creature with blindsight can perceive its surroundings without relying on sight, within 60 ft thanks to your dragonic ancestry. 

You can use your action to exhale destructive energy that does 2d8 fire damage on a 30 feet cone or 60 feet line.. The damage increases to 3d8 at 5th level, 4d8 at 10th level, and 5d8 at 15th level.  After you use your breath weapon, you can’t use it again until you complete a short or long rest.

You use your dragonic wings to fly with speeds up to 60 ft for several hours. You can use your wings again after a short or long rest.

Your skin is covered in hard dragon scales that gives you +2 to AC. 

Darkvision is the extraordinary ability to see with no light source at all, out to a range specified for the creature. Thanks to your demonic heritage, you have superior vision in dark and dim conditions up to 60 ft.

Due to your draconic ancestry you gain the use of the frightful presence ability. Whenever you attack or charge, all opponents within a radius of 30 feet who have fewer levels or Hit Dice than you become shaken for a number of rounds equal to 1d6 + your Charisma modifier. The effect is negated by a Will save (DC 10 + 1/2 your character level + your Charisma modifier). A successful save indicates that the opponent is immune to your frightful presence for 24 hours. This ability can't affect creatures with an Intelligence of 3 or lower, nor does it have any effect on dragons.

Due to your demonic ancestry you manifest spell like abilities according to your character's level.  The stronger the ability the less it can be used. The saving throw (if any) against a spell-like ability is:

10 + the level of the spell the ability resembles or duplicates + the creature’s Charisma modifier.

Level 1: Eldritch Blast, Create Bonfire, Control Flames, Detect Magic (5 times per short or long rest)

Level 3: Darkness, Alter self, Flame blade, Scorching ray, Dispel Magic (5 times per short or long rest)

Level 5: Blink, Fireball, Phantom steed, Remove curse, Summon lesser demons (4 times per short or long rest)

Level 7: Hell hound (Faithful hound spell), Summon Greater Demon (4 times per short or long rest)

Level 9: Far Step, Unholy weapon ( like holy weapon) (3 times per short or long rest)

Level 11: Planar Ally, Word of recall (3 times per short or long rest)

Level 13: Finger of Death, Regenerate, Teleport (2 times per short or long rest)

Level 15: Clone, Demiplane (1 times per short or long rest)

Level 17: Invulnerability, Meteor (1 time per long rest)

After a Xar'Athur has proven itself by overcoming various challenges and gaining experience (level 20) it will receive a trial. If they pass the trial given by the Xar'Athur Shaman they can take part in the ritual of the True Form. When the ritual is completed the Xar'Athur will become a full grown dragon. Although they may look like a Mature adult (201-400 years old), Old (401-600), Very old (601-800) or Ancient dragon (801-1000) they also manifest some of their demonic traits on their dragon form. They can change form at will. After the ritual of the True Form they gain these feats:

Immortality: No penalties from ageing. They do not have a finite lifespan like other lifeforms. They can still die if they receive heavy damage.

Damage reduction: The damage you receive is reduced by 10 for everything except Holy.

Spell Resistance: When in dragon form you gain Spell Resistance of 10 + its character level.

Mighty Breath: You can now use your breath up to 4 times per day and its damage is increased by 3d8. The breath does fire damage on a 60 feet cone or 90 feet line.

Improved Initiative: You get a +4 bonus on initiative checks when in dragon form.

Augmented Critical: Your dragon form's bite threatens a critical hit on a natural attack roll of 18-20, dealing triple damage on a successful critical hit.

Bite, Claws, Tails damage increase: Your natural attacks have increased damage. Bite: 5d8 piercing damage, Claws: 5d8 slashing damage , Tail: 5d8 bludgeoning damage.  

Natural Armor increase: Your scales become even harder in your dragon form covering now your whole body. That gives you a +4 in natural armor.

Dragonic Toughness: While in dragon form you gain health equal to half your hit points. Your hit points reset to normal when you change back to your normal form.

Truesight: The Xar'Athur in its true form can see up to 90 feet in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceives the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane. 

Innate Spellcasting: A Xar'Athur in its true form can innately cast spells. The spells require no material components. The Xar'Athur bonus to hit with a spell attack is equal to its proficiency bonus + its Charisma bonus. The Xar'Athur's spell save DC equals 12 + its proficiency bonus + its Charisma modifier.

Multiattack: The Xar'Athur in dragon form can make use of its Frightful Presence to make three attacks: one with its bite and one with its claws and one with its tail. Bite: 5d8 piercing damage, Claws: 5d8 slashing damage , Tail: 5d8 bludgeoning damage.