An'Hauak, the Grey Wind

Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a tiny creature. The swarm can’t regain hit points or gain temporary hit points.

Flensing Wind. The first time An'Hauak enters a creature's space on a turn, or when a creature ends its turn in An'Hauak's space, it takes 5 piercing damage.

Sunlight Aversion. An'Hauak cannot enter any area of natural or magical sunlight. If any part of An'Hauak is caught in sunlight, that part is petrified until it is no longer in sunlight.

Immutable Form. An'Hauak is immune to any spell or effect that would alter its form.

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

Turn Resistance. An'Hauak has advantage on saving throws against any effect that turns undead.

Maw of the Storm (Mythic Trait; Recharges after a short or long rest). When An'Hauak is reduced to 0 hit points, it instantly recharges and uses its Weight of the Storm, and doesn’t die or fall unconscious. Instead, the shattered splinters of bone that have been destroyed in the fight against An'Hauak re-animate, rising to form an enormous (treat as Huge) gaping circular maw at the center of the storm. Within this maw are three (Medium) fractured skulls, eyes burning with a malevolent light. Each skull has an AC of 18 and 50 hit points, and can only move within the space of the maw. It is immune to bludgeoning, piercing, and slashing damage from nonmagical attacks, has none of An'Hauak's other damage immunities, resistances, or  vulnerabilities, and has immunity to all conditions. If a skull is forced to make a saving throw, treat its ability scores as 10 (+0), and ignore saving throw proficiencies. If any of the skulls are exposed to direct sunlight An'Hauak flees, leaving that skull behind. If An'Hauak finishes a short or long rest, the maw vanishes, and any destroyed skulls re-form. Any skulls left behind by An'Hauak count as destroyed and do not reform so long as they stay in direct sunlight. An'Hauak dies when all the skulls are destroyed.

Trespassers in the Storm. As an action on its turn, An'Hauak makes up to one boneshard attack against each creatures occupying its space.

Boneshard. Melee Weapon Attack: +14 to hit, reach 0 ft., one target in the swarm’s space. Hit: 18 (4d6 + 4) piercing damage, or 9 (2d6 + 2) piercing damage if An'Hauak has half of its hit points or fewer, plus 3 (1d6) necrotic damage.

Weight of the Storm (Recharge; 5-6, or 2-6 when An'Hauak's Mythic trait is active). An'Hauak attempts to pin every creature whose space it is occupying. Each creature must succeed on a DC 19 Strength saving throw or take 18 (4d6 + 4) piercing damage and be rendered prone and restrained as shards of bone pierce its arms and legs and pin them to the soil below. Any prone creature has disadvantage on this saving throw. As an action, the restrained creature can escape the pin by succeeding on a DC 19 Strength (Athletics) or Dexterity (Acrobatics) check. An'Hauak cannot use its Boneshard attack against any pinned targets unless its Mythic Maw of the Storm trait is active. Any creature restrained by this feature is no longer restrained if An'Hauak ceases to occupy that creature's space.

An'Hauak 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. An'Hauak regains spent legendary actions at the start of its turn.

Move. An'Hauak moves up to half its speed without provoking opportunity attacks.

Wind's Snare. An'Hauak forces one creature within its space to make a DC17 Dexterity saving throw or be knocked prone.

Maelstrom of Bone (Costs 2 Actions). Until the start of its next turn, An'Hauak makes the space it is occupying count as difficult terrain for all creatures other than itself.

If An'Hauak's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Maw of the Storm.

Attack. An'Hauak uses its Boneshard attack against one target within its space.

Face the Maw. If a creature is within the area of An'Hauak's Maw of the Storm, An'Hauak can turn the gaze of its skulls upon that creature. That creature must make a Wisdom saving throw or be paralyzed for one minute. The creature can repeat this saving throw at the end of each of its turns. Once a creature has succeeded on this saving throw, it cannot be affected by this feature again. The DC of this saving throw is dependent on the number of skulls An'Hauak has remaining.