Adult Purple Dragon

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. 

Fey Ancestry. The dragon has advantage on saving throws against being charmed, and magic can’t put the dragon to sleep.

Sunlight Sensitivity.  Being born of the Underdrak, it's venom is less potent due to the sunlight, all poison deals half damage. 

Blind Sight: As they are creatures from the Underdark  their sight has been replaced by their other senses. They are immune to being blinded 

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: +14 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 9) piercing damage plus 4 [rollable](1d8); Each target that is bitten has to make a constitution saving throw. On a fail they take "2d6 +3": "poison damage" and gain the condition "posioned", on a successful throw they take half damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage. Each target hit by their claws has to make a constitution saving throw. On a fail they take "2d6 +3": "poison damage" and gain the condition "posioned", on a successful throw they take half damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 9) bludgeoning damage. Each target hit by the Tail has to make a constitution saving throw. On a fail they take "2d6 +3": "poison damage" and gain the condition "posioned", on a successful throw they take half 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 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.

Force Breath (Recharge 5–6). The dragon exhales a beam of pure energy in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) Force damage and pushed back 30 feet on a failed save, or half as much damage on a successful one.

Blindness Deafness: As an action the dragon can cast Blindness Deafness at 3rd level with 30 feet range (The dragon can blind or deafen a foe. Choose two creatures that you are aware of within range to make a Constitution saving throw. If it fails, the target is either blinded or deafened (your choice) for the duration. At the end of each of its turns, the target can make a Constitution saving throw. On a success, the spell ends.

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 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Darkness (Costs 3 actions). Magical darkness spreads from a point the dragon chooses within 60 feet of it, filling a 15-foot-radius sphere until the dragon dismisses it as an action, uses this lair action again, or dies. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. If any of the effect’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.