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.

Multiattack. The Behemoth makes three melee attacks: two with its claws and one with its tail swipe.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 18 (2d10 + 8) [slashing] damage.

Tail Swipe. Melee Weapon Attack: DC 18 Dexterity saving throw, range 10 ft. radius. Hit: 12 (2d4 + 8) [bludgeoning] damage.

The Behemoth has 4 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.

Comet (Costs 1 Actions). The Behemoth roars to the sky and with its horns discharging orange lightning he calls forth a comet a random creatures position. The creature must make a DC 21 Dexterity saving throw or take 12 (2d4 +8) bludgeoning damage and pushed 10ft in a random direction from the point of impact. At the 5ft square of impact the comet shall remain as a shield to avoid the effects of Ecliptic Meteor where it will break after impact of Ecliptic Meteor or if the Behemoth hits it with an attack twice.

Charybdis (Costs 2 Actions).  The Behemoth casts a 15ft radius circle (30ft high) vortex in a location within 120ft that it can see. That area becomes difficult terrain for 1d4 +1 rounds, a creature that starts its turn in, or enters that area must make a DC 21 Strength saving throw or be knocked prone and take 24 (3d12 +6) slashing damage.

Ecliptic Meteor. Once the Behemoth reaches 50% hp it will cast this spell with the use of its next two turns. The Behemoth must then wait another two turns before being able to cast Ecliptic Meteor again.

Ecliptic Meteor (Costs 2 Turns). The Behemoth spends its turn standing Bipedal and staring to the sky emitting deep purple lightning from its horns. At the end of its next turn it finishes the spell and a huge meteor will impact at the Behemoths position. All creatures who are not hidden behind any fallen comets or protected by a spell that makes one immune or invulnerable to damage (Globe of Invulnerability, etc.) must make a DC 30 Constitution saving throw. A creature takes 90 (20d6 +30) fire damage and 60 (20d6) bludgeoning damage on a failed save, or half as much on a successful one.