Caster

Amphibious. Caster can breathe air and water.

Legendary Resistance (3/Day). If Caster fails a saving throw, he can choose to succeed instead.

Change Shape. Caster magically polymorphs into a humanoid or beast that has a challenge rating no higher than his own, or back into his true form. He reverts to his true form if he dies. Any equipment it is wearing or carrying is absorbed or borne by the new form.

Eldritch Master. Caster's Eldritch Blast has the Agonizing Blast (+5), Elderich Spear (300ft), and Repelling Blast (10ft), 

Innate Spellcasting. Caster is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Caster has the following spells:

(At WIll): Eldritch Blast (4 Beams)

Multiattack. Caster can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws.

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.

Frightful Presence. Each creature of the Caster's choice that is within 120 feet of him and aware of him 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 Caster's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). Caster uses one of the following breath weapons.

Lightning Breath. Caster exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Repulsion Breath. Caster exhales repulsion energy in a 30-foot cone. Each creature in that area must succeed on a DC 23 Strength saving throw. On a failed save, the creature is pushed 60 feet away from Caster.

caster 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. Cster makes a Wisdom (Perception) check.

Magic Attack. Caster casts a spell.

Wing Attack (Costs 2 Actions). Caster beats hiss wings. Each creature within 15 feet of him must succeed on a DC 24 Dexterity saving throw or take 16 (2d6 + 9) bludgeoning damage and be knocked prone. Caster can then fly up to half his flying speed.