Armor Class 22 (natural armor)
Hit Points 370 (19d20 + 171)
Speed 40 ft., fly 80 ft., climb 30 ft.
Saving Throws CON +17, INT +18, WIS +11
Skills Arcana +18, Insight +11, Perception +11
Damage Resistances cold, psychic, thunder
Damage Immunities fire, lightning
Condition Immunities charmed
Senses blindsight 60 ft., darkvision 120 ft., passive Perception 21
Languages Common, Draconic
Challenge 26
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.
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.
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.
Guildmaster's Guide to Ravnica, p. 241