Aerial Nyxborn Genius Sharklike Increased Threat Robot Skeletal Zombie Mythical Half-Dragon Tarrasque

Legendary Resistance (6/Day). If the tarrasque fails a saving throw, it can choose to succeed instead.

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

Reflective Carapace. Any time the tarrasque 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, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.

Siege Monster. The tarrasque deals double damage to objects and structures.

Blood Frenzy. The tarrasque has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Amphibious. The tarrasque can breathe air and water.

Unusual Nature. The tarrasque doesn't require air, food, drink, or sleep. 

Undead Fortitude. If damage reduces the tarrasque to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the tarrasque drops to 1 hit point instead.

Multi-Souled (Mythic Trait; Recharges after a Short or Long Rest). When the tarrasque is reduced to 0 hit points, it doesn’t die or fall unconscious  Instead, the damage creates cracks in its carapace, revealing a 10 souls, for each of its adjectives. The souls float around the tarrasque's head. Each soul has 135 hit points and an armor class of 25. They are immune to bludgeoning, piercing, and slashing damage from nonmagical attacks, and are immune to all conditions. If they are forced to make a saving throw, treat their ability scores as 10 (+0). If the tarrasque finishes a short or long rest, the carapace heals, any destroyed souls regenerate, and the souls are covered again. The tarrasque dies when all the souls are destroyed.

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: +21 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 22). Until this grapple ends, the target is restrained, and the tarrasque can't bite another target.

Claw. Melee Weapon Attack: +21 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +21 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +21 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 22 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 19 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 22 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.

Fire Breath (Recharge 5–6). The tarrasque exhales fire in a 90-foot cone. Each creature in that area must make a DC 31 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.

The tarrasque can take 6 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. The tarrasque makes one claw attack or tail attack.

Move. The tarrasque moves up to half its speed.

Swoop. The tarrasque moves up to half its flying speed.

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

Wing Attack (Costs 2 Actions). The tarrasque beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 31 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone . The tarrasque can then fly up to half its flying speed.

If the tarrasque's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Multi-Souled.

Rampage. The tarrasque makes two attacks: one with its tail and one with its bite.

Frightening Blast (Costs 2 Actions). The tarrasque uses its Frightful Presence.

Invoke Tyrannosaurus (Costs 2 Actions). The tarrasque roars, summoning 1d20 Tyrannosaurus rexes. They appear in unoccupied spaces within 120 feet of the tarrasque. They roll for initiative and are hostile to the tarrasque's enemies, but don't take commands otherwise. 

Flame Destroyer. (Costs 6 Actions) The tarrasque exhales fire in a 120-foot cone. Each creature in that area must make a DC 31 Dexterity saving throw, taking 203 (58d6) fire damage on a failed save, or half as much damage on a successful one.