Actions

Multiattack. The tempest makes two Thunderous Slam attacks.   Thunderous Slam. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 23 (4d6+9) thunder damage.   Lighting Storm (Recharge 6). Each creature within 120 feet of the tempest must make a DC 21 Dexterity saving throw, taking 27 (6d8) 6d8 lightning damage on a failed save, or half as much damage on a successful one. If a target's saving throw fails by 5 or more, the creature is also stunned until the end of its next turn. 

Legendary Actions

The tempest 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 tempest regains spent legendary actions at the start of its turn.    Move. The tempest moves up to its speed.   Lightning Strike (Cost 2 Actions). The tempest can cause a bolt of lightning to strike a point on the ground anywhere under its storm. Each creature within 5 feet of that point must make a DC 21 Dexterity saving throw, taking 16 (3d10) 3d10 lightning damage on a failed save, or half as much damage on a successful one.   Screaming Gale (Costs 3 Actions). The tempest releases a blast of thunder and wind in a line that is 300 feet long and 10 feet wide. Objects in that area take 22 (4d10) 4d10 thunder damage. Each creature there must succeed on a DC 21 Dexterity saving throw or take 22 (4d10) 4d10 thunder damage and be flung up to 60 feet in a direction away from the the line. If a thrown target collides with an immovable object (such as a wall or floor) or another creature, the target takes 3 (1d6) 1d6 bludgeoning damage for every 10 feet it was thrown before impact. If the target collides with another creature, that other creature must succeed on a DC 19 Dexterity saving throw or take the same impact damage and be knocked prone.