Legendary Resistance (4/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Colossal. The dragon is considered Colossal sized (on grid: 60 ft. by 60 ft. or larger). This means that its hit dice are d20 + 5 [Example: 2d20 + (5 x CON modifier) + (2 x CON modifier)].

Building Charge. When the dragon rolls to recharge its lightning breath, creatures within 10 feet of it take 3 (1d6) lightning damage. If the dragon succesfully recharges its lightning breath, creatures within 5 feet of it take an additional 3 (1d6) lightning damage.

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: +19 to hit, reach 20 ft., one target. Hit: 25 (4d6 + 11) piercing damage + 15 (6d4) lightning damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 22 (2d10 + 11) slashing damage + 1 lightning damage.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 24 (2d12 + 11) bludgeoning damage.

Horn. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 20 (2d8 + 11) piercing damage + 13 (2d12) lightning 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 23 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.

Lightning Breath (Recharge 5–6). The dragon exhales lightning in a 125-­-foot line that is 10 feet wide. Each creature in that line must make a DC 26 Dexterity saving throw, taking 110 (20d10) lightning damage on a failed save, or half as much damage on a successful one.

Innate Spellcasting. The dragon's spellcasting ability is Charisma (spell save DC 23, +16 to hit with spell attacks). The dragon can innately cast the following spells, requiring only verbal components:

At Will: thaumaturgy, lightning lure, magic missile, create or destroy water, shield

2/day each: major image, thunder step, slow, arcane eye, wall of sand, telekinesis, project image, teleport, forcecage

5/day: chain lightning, programmed illusion, control weather, project image, etherealness

3/day: antimagic field, demiplane

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.

Swift Attack. The dragon makes a tail or claw attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 20 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.