Cubic Tarrasque

Psudo Legendary Resistance (3/Day). If the tarrasque fails a saving throw, choose to reroll it.

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.

Multiattack. The cubic tarrasque can use its Frightful Presence. It then makes four attacks: one with its bite, two with its claws, one with its horns. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit:  (4d8 + 8) piercing damage. If the target is a creature, it is grappled (escape DC 18). Until this grapple ends, the target is restrained, and the cubic tarrasque can't bite another target.

Claw. Melee Weapon Attack: +17 to hit, reach 10ft., one target. Hit:  (4d4 + 8) slashing damage.

Horns. Melee Weapon Attack: +17 to hit, reach 10ft., one target. Hit: (4d10 + 8) piercing damage.

Frightful Presence. Each creature of the cubic tarrasque's choice within 60 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 cubic 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 cubic tarrasque's Frightful Presence for the next 24 hours.