Legendary Resistance (3/Day). If Zirthon fails a saving throw, it can choose to succeed instead.

Magic Resistance. Zirthon has advantage on saving throws against spells and other magical effects.

Titanic Scales. The scales of Zirthon grant it immunity to all forms of damage save for lightning. If Zirthon takes lightning damage, its armor class is reduced to 22 for 1 minute and drops to resistance to all damage type for 1 minute. This does not affect its immunity to poison and acid.

Multi-Mind. Due to Zirthon's multiple minds, it cannot be affected by any mind-altering magic. In addition, it can concentrate on up to two spell effects simultaneously.

Innate Spellcasting. Zirthon's innate spellcasting ability is Wisdom (spell save DC 23). He can innately cast the following spells, requiring no material components: At will: earthquake, dispel magic; 3/day each: power word stun, destructive wave (necrotic), control weather.

Elder Rage. When Zirthon drops to half hit points or below, it releases a devastating roar. All creatures within 300 feet make a DC 24 Wisdom saving throw or become frightened for 1 minute. A creature that succeeds is immune to this ability for 24 hours.

Multiattack. Zirthon makes five attacks: One with its Bite, three with his colossal arm, and one with its tail. It can replace a bite attack with Massive devour if a creature smaller than it is in it's mouth.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the Zillo can’t bite another target.

Colossal Arm. Melee Weapon Attack: +19 to hit, reach 30ft., one target. Hit: 52 (8d12 + 12) bludgeoning damage. 

Tail Sweep. Melee Weapon Attack: +19 to hit, reach 20ft., all creatures in a 40-foot cone. Hit: 65 (10d10 + 12) bludgeoning damage. Targets must succeed on a DC 24 Strength saving throw or be pushed back 30 feet and knocked prone.

Massive Devour. Zirthon attempts to devour a creature that is in it's mouth. The target creature must succeed on a DC 25 Dexterity saving throw, or be swallowed by Zirthon, taking 55 (10d10) acid damage at the start of each of Zirthon’s turns. If the creature deals 50 damage while inside, it can free itself and emerge prone.

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

Thunderous Step. Zirthon moves up to half its speed. Each time it moves through a space occupied by a creature, that creature takes 22 (4d10) thunder damage and is pushed 10 feet back if it fails a DC 24 Strength saving throw.

Raze. Zirthon releases a powerful burst of energy from its body. All creatures within 30 feet must make a DC 23 Dexterity saving throw or take 48 (6d12) force damage.

Regenerative Surge (Costs 2 Actions). Zirthon regains 60 hit points. If struck by lightning in the past round, it regains an 30 hit points instead.