Heat Regeneration.

If the temperature around it is 100 degrees Fahrenheit or higher, Aurnozci regains 15 hit points at the start of its turn. If it takes cold or radiant damage, this trait doesn't function at the start of its next turn. Aurnozci dies only if it starts its turn with 0 hit points and doesn't regenerate.

Legendary Resistance (3/Day).

If Aurnozci fails a saving throw, it can choose to succeed instead.

Magic Resistance.

Aurnozci has advantage on saving throws against spells and other magical effects.

Actions

Multiattack.

Aurnozci makes one Bite attack and two Tail attacks.

Bite.

Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 21 (2d12 + 8) slashing damage plus 9 (2d8) fire damage.

Tail.

Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Mucus Spray (Recharge 5–6).

Aurnozci sprays mucus in a 60-foot cone. Each creature in that cone must make a DC 20 Dexterity saving throw, taking 42 (12d6) acid damage on a failed save, or half as much damage on a successful one.

Spellcasting.

Aurnozci casts one of the following spells, requiring no material components and using Wisdom as the spellcasting ability (spell save DC 20):

At will: Detect Magic, Heat Metal (7th-level version)
2/day each: Darkness, Dispel Magic, Hold Person

Legendary Actions

Aurnozci 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. Aurnozci regains spent legendary actions at the start of its turn.

Move. Aurnozci moves up to its speed.
Tail. Aurnozci makes one Tail attack.
Cast a Spell (Costs 2 Actions). Aurnozci uses Spellcasting.
Conflagration (Costs 2 Actions). Flames momentarily surround Aurnozci. Each creature within 15 feet of Aurnozci must make a DC 20 Dexterity saving throw, taking 18 (4d8) fire damage on a failed save, or half as much damage on a successful one.

Lair Actions

On initiative count 20 (losing initiative ties), Aurnozci can take one of the following lair actions; Aurnozci can't take the same lair action two rounds in a row:

Cocoon. Aurnozci targets one Medium or smaller creature it can see within 60 feet of itself and encases the creature in a constricting cocoon. The target must succeed on a DC 23 Strength saving throw or have the restrained condition for 1 minute. A target that starts its turn restrained in this way takes 9 (2d8) bludgeoning damage. A creature can use its action to try to tear apart the cocoon, ending the cocoon's effect with a successful DC 23 Strength (Athletics) check.
Insect Swarm. Aurnozci creates a 30-foot-radius sphere of stinging wasps centered on a point within 300 feet of itself. The sphere spreads around corners. The sphere remains for 10 minutes or until Aurnozci uses this lair action again. The sphere's area is heavily obscured and is difficult terrain. When the sphere appears, each creature in it must make a DC 20 Constitution saving throw, taking 16 (3d10) piercing damage and 16 (3d10) poison damage on a failed save, or half as much damage on a successful one. A creature must also make this saving throw when it enters the sphere's area for the first time on a turn or starts its turn there.

Regional Effects

Xulregg is an oppressively muggy realm known for the following regional effects:

Boiling Water. Bodies of water on Xulregg perpetually boil, and even bottled liquids steam when exposed to the layer's atmosphere. Being immersed in water there doesn't confer resistance to fire damage, and a creature that starts its turn in water takes 7 (2d6) fire damage. A creature that imbibes a liquid (such as a potion) takes 3 (1d6) fire damage.
Fast Metamorphosis. On Xulregg, eggs, larvae, and pupae all develop and metamorphose at ten times their usual rate, regardless of sustenance or any other growing conditions.

If Aurnozci dies, these effects fade over the course of 1d10 days.