Azathoth

Legendary Resistance (10/Day). If Azathoth fails a saving throw, it can choose to succeed instead.

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

Reflective Aura. Any time Azathoth is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the tarrasque is unaffected. On a 6, Azathoth is unaffected, and the effect is reflected back at the caster as though it originated from Azathoth turning the caster into the target.

Multiattack. Azathoth can use its Frightful Presence. It then makes seven attacks: one with its bite, five with its tentacles, two with its horns, and one with its tail. It can use its Swallow instead of its bite.

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 Azathoth can't bite another target.

Tentacles. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of Azathoth's choice within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if Azathoth is within line of sight, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Azathoth's Frightful Presence for the next 24 hours.

Surround With Void. Azathoth makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is surrounded by the Void, and the grapple ends. While surrounded by the Void, the creature is blinded and restrained, it has total cover against attacks and other effects outside Azathoth, and it takes 56 (16d6) acid damage at the start of each of the tarrasque's turns.

If Azathoth takes 60 damage or more on a single turn from a creature inside it, Azathoth must succeed on a DC 30 Constitution saving throw at the end of that turn or regurgitate all surrounded creatures, which fall prone in a space within 10 feet of Azathoth. If Azathoth dies, a surrounded creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Star-Mauler. 20th Level Spell: +20 to hit, reach ∞ ft., all available targets. Hit: 20d20 + 50 necrotic damage. All targets hit must pass a Constitution Saving Throw (DC 20), on a failed save,  the target is knocked unconscious instantly, on a passing save, the target receives half of their current hit-points in damage (after regular damage calculations) and is frightened.

Universal Strangle. ∞ Level Spell: +∞ to hit, reach ∞ ft., all available targets. Hit: 10d100 + 500 necrotic damage. Azathoth  All targets hit must pass a Constitution Saving Throw (DC 20), on a failed save,  the target is knocked unconscious instantly, on a passing save, the target receives half of their current hit-points in damage before damage calculations.

Attack. Azathoth makes one tentacle attack or tail attack.

Move. Azathoth moves up to half its speed.

Chomp (Costs 2 Actions). Azathoth makes one bite attack or uses its Swallow.