Ancient Green Dragon

Amphibious. The dragon can breathe air and water.

Hallucinogenic Gas. After the green dragon exhales its Poison Breath, wisps of poisonous gas laced with the alchemical taint that permeates the dragon's body linger around it. While the dragon's Poison Breath is unavailable, each creature that starts its turn within 30 feet of the dragon must succeed on a DC 21 WIS save or be terrified until the start of the creature's next turn, as the gas causes the creature to experience frightful hallucinations. A terrified creature is frightened, and its speed is reduced to 0.

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

Multiattack. The dragon uses its Frightful Presence. It then makes one Bite attack and two Claw attacks.

Bite. _Melee Weapon Attack:_ +15 to hit, reach 15 ft., one target. _Hit:_ 19 (2d10+8) piercing damage plus 10 (3d6) poison damage.

Claw. _Melee Weapon Attack:_ +15 to hit, reach 10 ft., one target. _Hit:_ 15 (2d6+8) slashing damage.

Tail. _Melee Weapon Attack:_ +15 to hit, reach 20 ft., one target. _Hit:_ 17 (2d8+8) 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 18 WIS save or be frightened for 1 minute. A creature can repeat the save at the end of each of its turns, ending the effect on itself on a success. If a creature's save is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Poison Breath (Recharge 5-6). The dragon exhales poisonous gas in a 90-foot cone. Each creature in that area must make a DC 21 CON save, taking 84 (24d6) poison damage on a failed save, or half as much damage on a successful one.

Detect. The dragon automatically detects every creature and trap within 30 feet of it, pinpointing the current location of each.

Tail Attack. The dragon makes a Tail attack.

Elemental Roar (Costs 2 Actions). The dragon roars at up to two creatures of its choice within 120 feet of it. Each target must succeed on a DC 18 CHA save or be vulnerable to poison damage until the end of its next turn. A target resistant to poison damage isn't resistant to it for the duration. A target immune to poison damage is still immune to it, even if the target fails the save.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 22 DEX save or take 15 (2d6+8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed without provoking opportunity attacks.