Amphibious. The dragon can breathe air and water.

Know Alignment (Good/Lawful). Abyssal Dragons are able to automatically detect if anyone in their presence within 120 feet are of Good or Lawful alignment.

Aura of Primal Emotion: Abyssal Dragons emotions are at their most raw and primal state. All Abyssal Dragons project one of the following emotions in a 60 foot radius.  All mortals(creatures that are not dragons, elementals, fey, constructs, or undead) must make a Wisdom saving throw (DC 21) at the beginning of each of their turns, or become afflicted by the emotion. If the auras of multiple Abyssal Dragons overlap, only the strongest takes effect, and if multiple Abyssal Dragons with the same power  have overlapping auras, those within them only need to save once to ignore both of them. This is considered a mind-affecting ability.

• Anger: Effect is identical to the Rage spell.
• Arousal: Effect is identical to the Charm Person/Monster spell(s).

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

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) acid damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Smite Good. Twice per day, an Abyssal Dragon can make a normal attack to deal an additional damage equal to its HD total (maximum +20) against a good target.

Smite Law. As Smite Good, but against lawful target.

 Abyssal Breath (Recharge 5-6). The dragon exhales a horrible gas in a 60 foot cone, infused with the madness of the blackest souls consigned to the Abyss. Each creature in the cone must make a DC 18 Wisdom saving throw, taking 6 (2d6) Wisdom damage on a failed save, and half as much damage on a successful one. This Wisdom damage can be healed with a Long Rest, Greater Restoration, or Wish spells.

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 16 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.

Breath Weapons (Recharge 5-6). The dragon uses one of the following breath weapons:

Poison Breath. The dragon exhales poison in a 90-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 66 (12d10) poison damage on a failed save, or half as much damage on a successful one.

Enervation Breath. The dragon exhales gas in a 60-foot cone. Each creature in that area must succeed on a DC 19 Constitution saving throw or have disadvantage on Constitution checks and Constitution saving throws 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.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher 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 dragon's choice).
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, 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.

The dragon 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 2 Actions). The dragon beats its wings. Each creature within 10 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.