Magic Resistance. The adamantoise has advantage on saving throws against spells and other magical effects.
Reflective Carapace. Any time the adamantoise 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 adamantoise is unaffected. On a 6, the adamantoise is unaffected, and the effect is reflected back at the caster as though it originated from the adamantoise, turning the caster into the target.
Siege Monster. The adamantoise deals double damage to objects and structures.

Adamantine Resistance. If the adamantoise is not currently stunned or  prone, all critical hits became regular hits.

Pain threshold. If the adamantoise takes more than 100 damage in a single turn, it is knocked prone and stunned until the start of its next turn. These conditions override its natural condition immunities.

Multiattack. The adamantoise can use its Frightful Presence. It then makes three attacks: one with its tusks and two with its stomp. If it cannot make any stomp attack, it may instead make one more tusk attack.
Stomp. Melee Weapon Attack: +16 to hit, reach 10ft., one target. Hit: 28 (4d8 + 8) bludgeoning damage. All targets within 40 ft. of the target who are touching the ground make a DC 18 strength saving throw. The stomped target and the additional targets take 2d10 bludgeoning damage and half damage on a successful save.
Tusks. Melee Weapon Attack: +16 to hit, reach 10ft., one target. Hit: 32 (3d10 + 8) piercing damage.
Frightful Presence. Each creature of the adamantoise's choice within 120 feet of it and aware of it must succeed on a DC 15 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 adamantoise 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 adamantoise's Frightful Presence for the next 24 hours.