Ancient mist dragon

Mist form. At will the mist dragon can enter an effect identical to gaseous form as a bonus action. 

Amphibious. The mist dragon can breathe air and water. 

Innate Spellcasting. The ancient mist dragon's innate spellcasting ability is Wisdom (spell save DC 18). The ancient mist dragon can innately cast the following spells, requiring no material or semantic components:

3/day each: fog cloud, gust of wind, sleet storm, wind wall

1/day each: control water, control weather 

Spellcasting. The Ancient mist dragon is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). The Ancient mist dragon has following sorcerer spells prepared:
Cantrips (at will): acid splash, dancing lights, mage hand, message, minor illusion, resistance
1st level (4 slots): comprehend languages, feather fall, thunderwave, 
2nd level (3 slots): blindness/deafness, detect thoughts
3rd level (3 slots): blink, counterspell 
4th level (3 slots): freedom of movement, storm sphere
5th level (3 slots): cloudkill, commune
6th level (2 slots): mass suggestion
7th level (2 slots): plane shift
8th level (3 slots): illusory dragon
9th level (3 slots): wish

Multi attack. The ancient mist dragon makes three attacks and uses its frightful presence. One with its bite and two with its claws. 

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., 1 target. Hit: 17 (2d10 + 7) piercing damage plus 8 (2d8) fire damage.  

Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 15 (2d8 + 7) slashing damage. 

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., 1 target. Hit: 15 (2d8 + 7) 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 17 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.

Steam breath. The mist dragon exhales a cone of scalding steam up to 80ft. Any creature within the radius of the breath must make a dc 17 constitution saving throw or take 78 (13d12) fire damage. On a successful save the target takes half damage. 

Caustic slime breath. The  mist dragon exhales a 10ft wide 90ft long line of caustic slime and any creature caught within the radius must make a dc 17 constitution saving throw or be poisoned for 1 minute. 

The ancient mist 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 ancient mist dragon regains spent legendary actions at the start of its turn.

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

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

Blinding mist (Costs 2 Actions). The mist dragon chooses an area of 20ft by 20ft within 120ft of themselves and that area is now obscured by mist anyone that isn't the dragon within this mist is counted as blind.