Flexing coil. Creatures that see the emerald dragon for the first time within 24 hours must make a DC 15 perception check or the dragon has advantage on attacking that creature for the first round of combat. 

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

Innate Spellcasting (Psionics). The adult emerald dragon's innate spellcasting ability is Intelligence (DC 16). The adult emerald dragon can innately cast the following spells, requiring no material components:

At will: fog cloud, greater invisibility, identify, legend lore, plane shift, suggestion (verbal only required)

Multi attack. The adult emerald dragon uses its frightful prescience then makes a bite attack and two claw attacks. 

Bite. Melee Weapon Attack: +5 to hit, reach 10 ft., 1 target. Hit: 10 (2d8 + 2) piercing damage. 

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 6 (2d4 + 2) slashing damage. 

Tail. Melee Weapon Attack: +5 to hit, reach 10 ft., 1 target. Hit: 8 (2d6 + 2) bludgeoning damage. 

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 15 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.

Sonic vibration breath. The dragon exhales a wave of sonic vibrations. Each creature within a 45ft cone must make a DC 13 Constitution saving Throw or be knocked unconscious for 1d4 minutes. On a successful save the target is deafened for 1d4 minutes. To wake a sleeping creature a character can use their bonus action to wake them allowing the creature to make the save again, other gem dragons are immune to this breath weapon. 

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 (Costs 1 Action). The dragon makes a Wisdom (Perception) check.

Tail Attack (Costs 1 Action). The dragon makes a tail attack.

Hypnotic shimmer (Costs 2 Actions). Each creature that can see the emerald dragon within 60ft must make a DC 15 Wisdom saving throw or be stunned until the end of their next turn.