Ancient Cockatrice

Legendary Resistance. The Ancient Cockatrice has 4 legendary resistances

Magic Resistance. The Ancient Cockatrice has advantage on all Spell saving throws

Multiattack. The Ancient Cockatrice can make 2 bite attacks and 2 Tail attacks.

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one creature. Hit: 54 (8d10 + 10) piercing damage, and the target must succeed on a DC 22 Constitution saving throw against being magically petrified. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified for 240 hours.

Tail. melee Weapon Attack: +11 to hit, reach 15 ft., one creature. Hit:  66(9d12 +8) Slashing damage.

Petrification Breath (recharge 6). The Ancient Cockatrice in haise its magical venom and exhales a 20ft cone of Petrifying breath. All creatures in the cone must make a dc Con save of 20, on a failure they become magically petrified turning them and their equipment to stone for 1 year.

Putrid breath (recharge 4-6). The Ancient Cockatrice exhales a putrid odor from its gut of all the things its eaten. All creatures within 30 ft of the cockatrice must make a constitution saving throw of 2. On failure they take 100(40d4) poison damage and are poisoned for 1 round, or half as much damage on a successful save.

The Ancient Cockatrice has a total of 4 legendary actions

Bite (one legendary action). The ancient cockatrice makes one Bite attack

Tail (one legendary action). The ancient cockatrice makes one Tail attack

Brood Calling (4 legendary actions). The Ancient cockatrice summons 2d8 + 4 Cockatrices to aid him

When the Ancient Cockatrice reaches o hitpoints it regains to max hp.  The Ancient cockatrice has 2 Mythic actions.

Bone Plating. The Ancient Cockatrice's Ac becomes 22

Stunning Stare (2 Mythic Actions). The Ancient cockatrice can stare at one creature as a bonus action and have advantage on one attack on the same turn.

Protection (1 Mythic Action). The Ancient Cockatrice can whenever an attack might hit the Ancient cockatrice, He can send a cockatrice to take the hit instead.