Akadi

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

Flyby. She doesn’t provoke opportunity attacks when she flies out of an enemy’s reach.

Spell Immunity and Resistance: Immune to 5th level spells or lower, rolls with advantage for all others.

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

Legendary Immunity. She is immune to lightning, thunder, and force damage from wind or sound attacks.

Living Storm.  She is always at the center of a storm and can control it at will 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.

Spellcasting. She is an 24th-level spellcaster. Her spellcasting ability is charisma (spell save DC 24, +15 to hit with spell attacks). Metamagic options: Carefull Spell, Heightened spell, Quickened spell, Subtle spell, and Twinned Spell. She has 24 sorcery points. She doesn't spend points on elemental based spells. She has the following sorcerer spells prepared:

Cantrips (at will): lightning lure, light, mage hand, prestidigitation, shocking grasp, thunderclap

1st level (6 slots): detect magic, hunter's mark, magic missile, shield, shield of faith

2nd level (5 slots): detect thoughts, mirror image, misty step, vortex warp

3rd level (5 slots): counterspell, lightning bolt, sending, call lightning

4th level (5 slots): fire shield, greater invisibility, polymorph, summon elemental

5th level (5 slots): Bigby’s hand, geas, wall of force, conjure elemental

6th level (3 slot): flesh to stone, globe of invulnerability, chain lightning

7th level (2 slot): finger of death, resurrection, teleport

8th level (2 slot): feeblemind, mind blank, tsunami

9th level (2 slot): imprisonment, power word kill, storm of vengeance

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

Multiattack. Akadi makes two attacks with her Flail or thunderous slam, or one spell attack. 

Flail. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 22 (4d6 + 8) bludgeoning damage plus 27 (6d8) radiant damage.

Thunderous Slam. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 30 (6d6 + 18) thunder damage.

Lightning Storm (Recharge 5-6). All other creatures within 120 feet of the Akadi must each make a DC 23 Dexterity saving throw, taking 40 (8d8) lightning damage on a failed save, or half as much damage on a successful one. Ignores all defense's, resistance and immunity is treated as resistance. If a target’s saving throw fails by 5 or more, the creature is also stunned until the end of its next turn.

Spell casting. Akadi can cast a cantrip or any lightning elemental spell as a bonus action.

Storm's Fury. When she is hit by a melee attack, she can use her reaction to deal lighting damage equal to her sorcerer level 24. The attacker must also make a Strength Saving throw against her spell DC 24. On a failed save, the attacker is pushed in a staight line up to 20 feet away from her.

She 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. She moves up to her speed.

Lightning Strike (Costs 2 Actions). She 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 24 Dexterity saving throw, taking 20 (4d10) lightning damage on a failed save, or half as much damage on a successful one.

Screaming Gale (Costs 3 Actions). She releases a blast of thunder and wind in a line that is 1 mile long and 20 feet wide. Objects in that area take 30 (6d10) thunder damage. Each creature there must succeed on a DC 24 Dexterity saving throw or take 30 (6d10) thunder damage and be flung up to 60 feet in a direction away from the line. If a thrown target collides with an immovable object, such as a wall or floor, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown before impact. If the target would collide with another creature instead, that other creature must succeed on a DC 21 Dexterity saving throw or take the same damage and be knocked prone.