Shape Change. Aerynthyr has the ability to change her form into a medium humanoid as an action. The player chooses this form, and the Aerynthyr may only assume this same form each time. In this form Aerynthyr may use her Claw and Breath attack, loses her fly speed, and her walking speed is 30 feet. Her stats otherwise remain the same.
Supple Wings. Can Dash as a bonus action, and any stealth checks made while mounted gain advantage.
Multiattack. Once the Rider has gained access to Extra Attack, the Dragon Companion may use any of the following attacks once per turn, following the rules of attacking as a Dragon Companion.
Bite. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 3 (1d6) cold damage.
Tail. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 8 (1d10+3) bludgeoning damage.
Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6+3) slashing damage.
Cold Breath (1/rest). Aerynthyr exhales an icy blast in a 25-foot cone. Each creature in that area must make a DC (8 + proficiency + Dragon's Constitution Modifier) Constitution saving throw, taking 28(8d6) cold damage on a failed save, or half as much damage on a successful one.
Dragon Maneuvers. If the target of a Dragon Maneuver is forced to make a saving throw against the Dragon's Save DC, the DC is 8 + Dragon's Strength Modifier + Dragon's Proficiency.