Air Form. The tempest can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Flyby. The tempest doesn’t provoke opportunity attacks when it flies out of an enemy’s reach.

Living Storm. The tempest is always at the center of a storm 1d6 + 4 miles in diameter. Heavy precipitation in the form of either rain or snow falls there, causing the area to be lightly obscured. Heavy rain also extinguishes open flames and imposes disadvantage on Wisdom (Perception) checks that rely on hearing.

In addition, strong winds swirl in the area covered by the storm. The winds impose disadvantage on ranged attack rolls. The winds extinguish open flames and disperse fog.

Siege Monster. The tempest deals double damage to objects and structures.

Multiattack. The tempest makes two attacks with its thunderous slam.

Thunderous Slam. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 17 (3d6 + 7) thunder damage.

Lightning Storm (Recharge 6). All other creatures within 80 feet of the tempest must each make a DC 18 Dexterity saving throw, taking 22 (5d8) 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.

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 (Costs 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 18 Dexterity saving throw, taking 13 (3d8) lightning damage on a failed save, or half as much damage on a successful one.