Ancient Two Headed Sea dragon Kaltrensa

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

Two-Headed. The dragon has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked  unconscious.
The dragon has two different heads an Aqua crystal and a Crimson fin. Each of these heads has 200 hit points and when a creature makes an attack roll against the Dragon they can choose which head they are attacking. If the dragon is forced to make a saving throw it only makes one even if both heads are in the affected area. 

Wakeful.  While the dragon sleeps, at least one of its heads is awake.

Multiattack. Enter the description for your action. The dragon can use its Frightful Presence. It then makes four bite attacks 2 with each head if they are available. The dragon can forgo one of its bite attacks to make a Wisdom perception check.

Aqua crystal head bite. Melee Weapon Attack: +16 to hit, reach 20 ft., 1 target. Hit: 20 (2d10 + 9) piercing damage, 4 (1d6) acid damage, and 4 (1d6) fire damage. 

Crimson fin head bite. Melee Weapon Attack: +16 to hit, reach 20 ft., 1 target. Hit: 20 (2d10 + 9) piercing damage, and 7 (2d6) psychic damage.

Aqua crystal head breath weapon (recharge 5-6). The dragon exhales a bright blue flame in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 35 (7d8) acid damage and 35 (7d8) fire damage on a failed save, or half as much damage on a successful one. Creatures that fail the save have blue flames stuck to them for (1d4+1) rounds. While the blue flames are stuck to a creature its AC is reduced by 1 for each time they have failed the save, as its armor, clothing, or skin is dissolved by the sticky flames. Armor that grants fire or acid resistance is immune to this reduction. 

Crimson fin head breath weapon (recharge 5-6). The dragon exhales a crimson-red gas cloud in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, or be affected by a 5th level dispel magic spell dispelling any spells affecting them or that they are concentrating on. Other magical effects in the cloud area are also affected by the dispelling. All items inside the cloud area are affected by an anti-magic field until they are no longer in the cloud. The cloud is light obscured and ranged attack rolls have disadvantage if they travel through the cloud. The cloud last for (1d4+1) rounds. The cloud lays where it was spread until it ends or a wind of moderate or greater speed (at least 10 miles per hour) disperses it.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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.

Bite (Costs 1 Action).  The dragon makes a bite attack.

Dive (Costs 1 Action). The dragon dives deep into the water moving half its speed. Creatures within 10ft of the dragon in the water are pulled with it unleash they have a swim speed, Or are affected by the water walk spell.

Back flop (Costs 2 Actions). The dragon jumps into the air before falling back into the water creating a large wave in the water. Creatures in or on the water surface within 120ft must make a  DC 24 Strength saving throw, taking 9 (2d8) cold damage and 9 (2d8) bludgeoning damage on a failed save, or half as much damage on a successful one. Creatures that fail the save are pushed 30ft with the wave. If the saving throw fails by 5 or more, the creature is pushed 60ft total.