Ancient chain golem

Legendary Resistance (3/Day). If the Ancient chain golem fails a saving throw, it can choose to succeed instead.

Magic Resistance. The Ancient chain golem has advantage on saving throws against spells and other magical effects.

Reflective Alloy. Any time the Ancient chain golem is targeted by a spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the Ancient chain golem is unaffected. On a 6, the is unaffected, and the effect is reflected back at the original caster as though it originated from the Ancient chain golem, turning the caster into the target.

Siege Monster. The Ancient chain golem deals double damage to objects and structures.

Multiattack. The Ancient chain golem can use its Frightful Presence. It then makes five attacks: one with its bite, two with its claws, one with its horns.

Smash. 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.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Frightful Presence. Each creature of the Ancient chain golem'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 Ancient chain golem 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 Ancient chain golem's Frightful Presence for the next 24 hours. 

Can cast on command chain lightning and 

The tarrasque 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. The tarrasque makes one claw attack or tail attack.

Move. The Ancient chain golem moves up to half its speed.