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

Immutable Form. The whirlwind is immune to any spell or effect that would alter its form.

Magic Resistance. The whirlwind has advantage on saving throws against spells and other magical effects.

Antimagic Susceptibility. The swarm is incapacitated while in the area of an antimagic field. If targeted by dispel magic, the swarm must succeed on a Constitution saving throw against the caster’s spell save DC or fall unconscious for 1 minute.

False Appearance. While the objects remain motionless, they are indistinguishable from a regular clutter of objects.

Whirlwind Form. As soon as a creature steps within 55', the objects animate and fly to surround the creature that caused them to animate in a whirlwind motion, causing an actual whirlwind to form around them. When the whirlwind forms, all creatures in its area must make a DC 18 Strength save. On a failed save, the creature takes 28 (5d8+6) bludgeoning damage (or 19 (3d8+6) bludgeoning damage if the whirlwind has half its hitpoints or fewer) and falls prone; on a successful save, the creature takes half that amount and doesn't fall. Any creature that starts its turn within the whirlwind's area is subject to the same effect.
In addition, the strong wind keeps fog, smoke, and other gases at bay. Small or smaller flying creatures or objects can't pass through the wall. Loose, lightweight materials brought into the wall fly upward. Arrows, bolts, and other ordinary projectiles launched at targets behind the wall are deflected upward and automatically miss. (Boulders hurled by giants or siege engines, and similar projectiles, are unaffected.) Creatures in gaseous form can't pass through it. The whirlwind's area is considered difficult terrain for any creature of size Large or smaller.

Multiattack. The whirlwind makes one grab and one throw attack.

Grab. Melee Weapon Attack: +10 to hit, reach 0 ft., one Large or smaller target in the swarm's space. Hit: 20 (4d6+6) bludgeoning damage, or 13 (2d6+6) bludgeoning damage if the whirlwind has half of its hit points or fewer, and the target is grappled (escape DC 18).

Throw. Ranged Weapon Attack: +10 to hit, range 50/200 ft., one grappled target. Hit: 28 (4d10+6) bludgeoning damage.
If the target is a creature, it must succeed on a DC 18 Strength saving throw or be knocked prone. If the thrown projectile is a creature, it shares this damage with the target, and must succeed on a DC 18 Dexterity saving throw or be knocked prone.