Ancalagon the red, aka the fire storm general

Amphibious. Ancalagon can breathe air and water.

Storm adept. Ancalagon instead of travelling by foot it can transform into storm of fire Instead but this takes one minute to do. The storm of fire has a movement speed 240 ft and it has the same HP as the Giant, but it can only be hurt by cold damage. If it wants to turn back into its giant form It takes a minute to transform back.

Regeneration. The Ancalagon regains 15 hit points at the start of its turn if it has at least 1 hit point.

Innate Spellcasting. Ancalagon spellcasting ability is Intelligence (spell save DC 20). Ancalagon can innately cast the following spells, requiring no material components:

At will: detect magic, fireball

3/day each: hold monster, wall of fire

Multiattack. The Ancalagon can use its Frightful Presence and makes four attacks: one with its bite, one with its claw, one with its mace, and one with its tail. 

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (3d8) fire damage.

Storm Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage plus 21 (6d6) lightning damage.

Claw. Melee Weapon Attack: +14to hit, reach 10 ft., one target. Hit: 17 (2d8+8) slashing damage + (2d8) lightning damage .

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (4d10+8) bludgeoning damage.

Frightful Presence. Each creature of Ancalagon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 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.

 Fire Breath (Recharge 5-6). Ancalagon exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one

Lightning Strike (Recharge 5-6). The Ancalagon hurls a magical lightning bolt at a point it can see within 500 feet of it. Each creature within 10 feet of that point must make a DC 24 Dexterity saving throw, taking 54 (17d8) lightning damage on a failed save, or half as much damage on a successful one.

Powerful attack. If Ancalagon the red hits a creature with an attack, he can use his Bonus action to make the creature make a strength DC 20 if the creature fails it is knocked 40ft back.

Teleport. Ancalagon magically teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see

Ancalagon 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. Ancalagon makes a Wisdom (Perception) check.

Tail Attack. Ancalagon makes a tail attack.