AO3, Champion of Ao

Spellcasting. AO3 is a 20th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 23, +18 to hit with spell attacks). AO3 has following Cleric spells prepared:
Cantrips (at will): Sacred flame, Guidance, Toll the dead, Word of radiance, Light, Resistance
1st level (4 slots): Shield of faith, Guiding bolt

2nd level (3 slots): Hold person, Enhance ability

3rd level (3 slots): Revivify, Water walk

4th level (3 slots): Banishment, Freedom of movement

5th level (3 Slots) Commune, Holy weapon

6th level (2 slots) Sunbeam (Cast from the mouth), Find the path

7th level (2 slots) Divine word, resurrection

8th level (1 slot) Sunburst (Also cast from the mouth)

9th level (1th) Gate

Multiattack. The tarrasque can use its Frightful Presence. It then makes five attacks: one with its bite, two with its claws, one 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 the tarrasque can’t bite another target.

Claw. 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 the tarrasque’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 the tarrasque 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 the tarrasque’s Frightful Presence for the next 24 hours.

Swallow. The tarrasque 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 swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque’s turns.

If the tarrasque takes 60 damage or more on a single turn from a creature inside it, the tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the tarrasque. If the tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Cleric of war: Make another attack using your bonus action. Can be used 5/day

AO3 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. The tarrasque regains spent legendary actions at the start of its turn.

Attack (1 Action): AO3 makes one claw attack or tail attack.

Move (1 Action): AO3 moves up to half its speed.

Prayer (1 Action): AO3 casts the cure wounds spell on 5th level

Rampage (2 Actions): Move your full movement speed around. If you walk over or through an enemy they take 6d6 bludgeoning damage.

When AO3 reaches 0 hitpoints his mythic actions activate. He gains the benefits of a long rest and can use the mythic actions below when they say they can be used. AO3 has 3 mythic actions

Chosen of Ao (Feature): You gain immunity to radiant and necrotic damage and ignore all damage resistances and immunities. All your attacks also become magical for the purpose of overcoming resistances.

Moment of clarity (1 Action): When someone makes you make a intelligence or wisdom saving throw you can give yourself a +5 to either one

Top of the chain (1 action): You cast the sunbeam spell without using a spell slot when someone comes within 5 feet of you