Legendary Resistance (3/Day).

If Niv-Mizzet fails a saving throw, he can choose to succeed instead.

Locus of the Firemind.

Niv-Mizzet can maintain concentration on two different spells at the same time. In addition, he has advantage on saving throws to maintain concentration on spells.

Magic Resistance.

Niv-Mizzet has advantage on saving throws against spells and other magical effects.

Master Chemister.

When Niv-Mizzet casts a spell that deals damage, he can change the spell's damage to cold, fire, force, lightning, or thunder.

Spellcasting.

Niv-Mizzet is a 20th-level Izzet spellcaster. His spellcasting ability is Intelligence (spell save DC 26, +18 to hit with spell attacks). He has the following wizard spells prepared:

Cantrips (at will): fire bolt, light, prestidigitation, ray of frost, shocking grasp
1st level (4 slots): detect magic, magic missile, shield, thunderwave, unseen servant
2nd level (3 slots): blur, enlarge/reduce, flaming sphere, hold person, scorching ray
3rd level (3 slots): counterspell, fireball, lightning bolt, slow
4th level (3 slots): confusion, dimension door, fabricate
5th level (2 slots): conjure elemental, polymorph, wall of fire, wall of force
6th level (1 slot): chain lightning, disintegrate, true seeing
7th level (1 slot): project image, reverse gravity, teleport
8th level (1 slot): control weather, maze, power word stun
9th level (1 slot): prismatic wall

Actions

Multiattack.

Niv-Mizzet makes three attacks: one with his bite and two with his claws.

Bite.

Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) piercing damage plus 14 (4d6) fire damage.

Claw.

Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 14 (2d4 + 9) slashing damage.

Tail.

Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 16 (2d6 + 9) bludgeoning damage.

Fire Breath (Recharge 5–6).

Niv-Mizzet exhales fire in a 90-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.

Legendary Actions

Niv-Mizzet 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. Niv-Mizzet regains spent legendary actions at the start of its turn.

Cantrip. Niv-Mizzet casts one of his cantrips.
Tail Attack. Niv-Mizzet makes a tail attack.
Wing Attack (Costs 2 Actions). Niv-Mizzet beats his wings. Each creature within 15 feet of him must succeed on a DC 25 Dexterity saving throw or take 14 (2d4 + 9) bludgeoning damage and be knocked prone. Niv-Mizzet can then fly up to half his flying speed.
Dracogenius (Costs 3 Actions). Niv-Mizzet regains a spell slot of 3rd level or lower.