Vael'Zarun, The Thunder Unbound

Innate Spellcasting. The couatl's spellcasting ability is Charisma (spell save DC 20). It can innately cast the following spells, requiring only verbal components:

At will: dispel magic, detect magic, detect thoughts

3/day each: [Tooltip Not Found], shape water, inflict wounds, lesser restoration, disguise self, poison spray, shield

1/day each: dream, [Tooltip Not Found], power word kill

Magic Weapons. The couatl's weapon attacks are magical.

Shielded Mind. The couatl is immune to scrying and to any effect that would sense its emotions, read its thoughts, or detect its location.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 8 (1d6 + 5) piercing damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned for 24 hours. Until this poison ends, the target is unconscious. Another creature can use an action to shake the target awake.

Constrict. Melee Weapon Attack: +6 to hit, reach 30 ft., one Huge or smaller creature. Hit: 10 (2d12 + 4){"diceNotation":"2d12+4","rollType":"damage","rollAction":"Constrict","rollDamageType":"bludgeoning"} bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained, and the couatl can't constrict another target.

Change Shape. The couatl magically polymorphs into a humanoid or beast that has a challenge rating equal to or less than its own, 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 couatl's choice).

In a new form, the couatl retains its game statistics and ability to speak, but its AC, movement modes, Strength, Dexterity, and other actions are replaced by those of the new form, and it gains any statistics and capabilities (except class features, legendary actions, and lair actions) that the new form has but that it lacks. If the new form has a bite attack, the couatl can use its bite in that form.

Lightning Breath (Recharge 5–6). The dragon exhales lightning in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Recharge 5–6). The dragon exhales lightning in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 17 Wisdom saving throw 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 dragon's Frightful Presence for the next 24 hours.

The Coutal can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 1 Bounus Action). The dragon beats its wings. Each creature within 40 feet of the dragon must succeed on a DC 20 Dexterity saving throw or take 14 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.