Amphibious. Zongkham can breathe air and water.

Legendary Resistance (4/Day or 5/Day in Lair). If Zongkham fails a saving throw, he can choose to succeed instead.

Lightning Absorption. Whenever Zongkham is subjected to lightning damage, he takes no damage and instead regains a number of hit points equal to the damage dealt.

Magic Weapons. Zongkham’s weapon attacks are magical.

Ol’ Dog o’ War (Mythic Trait; Recharges after a Long Rest). If Zongkham would be reduced to 0 hit points, his current hit point total instead resets to 500 hit points, he recharges his Lightning Breath, and he regains any expended uses of Legendary Resistance. Additionally, Zongkham can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating Zongkham after his Ol’ Dog o’ War activates.

Unusual Nature. Zongkham doesn't require food or drink.

Multiattack. Zongkham can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws. He can replace one attack with Repulsion Breath.

Bite. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 26 (3d10 + 10) piercing damage plus 16 (3d10) lightning damage.

Claw. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 20 (3d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +18 to hit, reach 25 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage, and the dragon pushes the target 10 feet away.

Frightful Presence. Wisdom Saving Throw: DC 25, each creature of Zongkham‘s choice within 150 feet and aware of him. Failure: The target is frightened for 1 minute and can repeat the saving throw at the end of each of its turns. Success or Failure: If a target’s saving throw is successful or the effect ends for it, the target is immune to Zongkham‘s Frightful Presence for the next 24 hours.

Lightning Breath (Recharge 5–6). Dexterity Saving Throw: DC 26, each creature in a 150-foot-long, 15-foot-wide Line. Failure: 99 (18d10) lightning damage. Success: Half damage.

Repulsion Breath. Strength Saving Throw: DC 26, each creature in a 60-foot cone. Failure: The target is pushed up to 60 feet away from the dragon and has the prone condition.

Spellcasting. Zongkham casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 25, +17 to hit with spell attacks):

At Will: Create or Destroy Water, Detect Magic, Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell), Shocking Grasp, Speak with Animals, Thaumaturgy

3/Day Each: Commune with Nature, Detect Thoughts, Lightning Bolt, Tidal Wave

1/Day Each: Chain Lightning, Control Water, Scrying, Watery Sphere

Zongkham 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. Zongkham regains spent legendary actions at the start of his turn.

Shocking Grasp. Zongkham casts Shocking Grasp against one target plus one additional target adjacent to the first target.

Wing Attack (Costs 2 Actions). Dexterity Saving Throw: DC 26, each creature within 15 feet. Failure: 20 (3d6 + 10) bludgeoning damage, and the target is knocked prone. Success: Half damage. Success or Failure: Zongkham can then fly up to half his flying speed.

If Zongkham’s Ol’ Dog o’ War trait has activated in the last hour, he can use the options below as legendary actions.

Electrified Tail. Zongkham makes a tail attack. It deals an extra 16 (3d10) lightning damage, and a creature must succeed on a DC 25 Constitution saving throw or be paralyzed until the end of its next turn.

Shattering Roar (Costs 2 Actions). Constitution Saving Throw: DC 26, each creature within a 150-foot-radius sphere of Zongkham. Failure: 26 (4d12) thunder damage, and the target is deafened and incapacitated until the end of its next turn. Success: Half damage.