Behemoth, The Legendary Beast

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

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

Siege Monster. Behemoth deals double damage to objects and structures.

Frightful Presence. Each creature within 120 feet of the Behemoth 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 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 Behemoth Frightful Presence for the next 24 hours.

Ecliptic Meteor. When Behemoth drops to 0 HP, he casts Ecliptic Meteor. The meteor falls 1 round after cast. The meteor has a 1 mile range, killing any creature caught in the blast instantly. The only way to block the blast is by hiding behind any comets that Behemoth cast earlier. After the blast, Behemoth falls dead. 

Multiattack.  Behemoth makes 2 Claw attacks and 1 Horn attack. Behemoth can then move up to half his speed without costing movement.  

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

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

Tail. Melee Weapon Attack: +19 to hit, reach 20ft centered on Behemoth. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Meteor. 300 ft. range, 10 ft. radius centered on target. Behemoth picks up to 3 targets he can see within range. Each target must make a DC 20 Dexterity save, taking 18(6d6) fire damage and 18(6d6) bludgeoning damage, or half on a success. A target who fails the save also gets set on fire, taking 4(1d8) fire damage at the start of each of their turns until an action is used to put it out.  

Charybdis (Recharge 5-6). 300 ft. range. Behemoth picks one target it can see within range. A spiraling tornado 20 ft. wide appears in that location for 3 rounds. A creature caught within the tornado must make a DC 20 dexterity save. Targets that fail take 25(5d10) bludgeoning damage and are knocked away 10 ft. and prone. Targets take half damage on a success, but are knocked out of the range of the tornado.

Thunderbolt (5 per short rest). 100 ft. line perpendicular to Behemoth. Lightning bolts blast down, hitting all targets in range. Each target makes a DC 20 dexterity save, taking 50(10d10) lightning damage and becoming paralyzed for 1 minute. A target can make a constitution save DC 20 to resist the effect at the end of each of it's turns. A successful save ends the effect. A successful save of Thunderbolt takes half damage and is not paralyzed. 

Comet. (4 per long rest) After taking 150 points of damage, Behemoth casts comet, dropping down a comet on 1 target Behemoth can see. The target makes a DC 20 dexterity save, taking 60(20d6) bludgeoning damage, or no damage on a success. The comet lands and stays on the field, It has an AC of 15 and HP of 100. If the comet drops to 0 HP it shatters and is useless. Behemoth casts this 4 times before he either completes a long rest or drops to 0 HP.

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 horn attack.

Pounce. The Behemoth jumps up to 40ft. Any creature within the space Behemoth lands in must succeed a DC 20 Dex save or take 30(10D6) bludgeoning damage and be knocked prone. Targets take half on a success and are not knocked prone. 

Burst. 5 ft. range. The Behemoth picks 3 20ft cubes that all connect together. Targets caught in the blast must make a DC 20 Dexterity save, taking 24(8d8) fire damage on a fail or half as much on a success.