Unhindered. The dragon's movement is unaffected by difficult terrain.

Amphibious. The dragon can breathe air and water.

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

Multiattack. The dragon can use its Calming Presence. It then makes three attacks: one with its bite and two with its tail. Or it can use it's Action to Constrict, or use a breathe attack. 

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 4) piercing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 20 ft., one target. Hit: 14 (2d8 + 4) bludgeoning damage.

Constrict. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature. Hit: 27 (6d6 + 6)bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the creature is restrained, and the dragon can't constrict another target.

Calming Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC20 Wisdom saving throw or become Calmed for 1 minute, and is unable to attack the dragon or be angry or upset at it in any fashion unless the dragon does them harm. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success, or, if the dragon harms them or an ally of their's, the effect is ended instantly. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Calming Presence for the next 24 hours.

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

Moonlit Breath. The dragon releases a beam of moonlight in a 60-foot cone. Each creature in that area must make a DC 20 Constitution saving throw, taking 66 (12d10) Radiant damage on a failed save, or half as much damage on a successful one.

A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spell's light.

Rain Calling Breath;  The Dragon exhales a cloud, that fills the air  within 20 miles of it for 3d8 hours. When this is done, the dragon can change the current weather conditions, which are determined by the DM based on the climate and season. The dragon can then change the weather one scale up or down every round for the duration of the breathe. When the spell ends, the weather gradually returns to normal.

When the dragon first changes the weather conditions, find the current condition on the following tables and change its stage by three, up or down.

Precipitation

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.

Constrict. The dragon Constricts.

Consume Negative Emotion; Pearl dragons have the unique ability to Consume/be empowered by different things, and convert the energy from that into pearls for their horde. Blue Pearl dragons Eat Anxiety, Fear, and Anger, leaving the individuals it feeds upon more calm and relaxed. The consumed emotions are then turned into pearls within the dragon, to be spat out into it's horde later. This has the same effect in combat as the "Calm person" Spell, (Save DC 19), and can end a Barbarian's rage.