Behemoth

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

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

Ecliptic Meteor. When the behemoth dies it calls down a meteor on top if itself that destroys its body. All creatures within 120 feet of the behemoth must make a DC 22 dexterity saving throw taking 140 (20d6 bludgeoning and 20d6 fire damage) on a failed save or half as much on a successful one. 

Multiattack. The behemoth 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. 

Bite. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 31 (4d10 + 9) piercing damage. 

Claw. Melee Weapon Attack: +18 to hit, reach 15ft., one target. Hit: 23 (4d6 + 9) slashing damage.

Horns. Melee Weapon Attack: +18 to hit, reach 10ft., one target. Hit: 31 (4d10 + 9) piercing damage plus 13 (3d8) lightning damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20ft., one target. Hit: 21 (4d6 + 9)bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the behemoth's choice within 120 feet of it and aware of it must succeed on a DC 22 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 behemoth 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 behemoth’s Frightful Presence for the next 24 hours.

Comet. (Recharge 5-6)  The behemoth calls down a comet on a place it can see within 120 feet of it. Each creature in a 20 foot radius sphere must make a DC 22 dexterity saving throw taking 84 (12d6 bludgeoning and 12d6 fire damage) on a failed save or half as much on a successful one.

The behemoth 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 behemoth regains spent legendary actions at the start of its turn.

Attack. The behemoth makes one claw attack or tail attack.

Thunderbolt (Costs 2 actions.) The behemoths horns spark with electricity. All creatures in an 30ft. cone in front of the behemoth must make a DC 22 dexterity saving throw taking 36 (8d6) lightning damage or half as much on a failed save.

Charybdis (Costs 3 Actions). The behemoth summons a whirlwind in a place it can see within 120 feet of it.