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

Magical Attacks. The zephyr demon's attacks are magical. 

Siege Monster. The zephyr demon does double damage to buildings and structures.

Tornado Form. The zephyr demon can appear as a huge, living tornado. While as a tornado it gains all its actions. If it is not appearing as a tornado, it is invisible and can only use its slam attacks and summon mephits actions. The zephyr demon also can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing. Any creature that it passes through takes 6 (2d6) bludgeoning damage. 

Multiattack. The zephyr demon makes two slam attacks.

Slam. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 18 (3d8 + 6) bludgeoning damage, as well as 9 (3d6) lightning damage

Whirlwind (Recharge 4–6). Each creature in the zephyr demon's space must make a DC 16 Strength saving throw. On a failure, a target takes 18 (4d8 + 3) bludgeoning damage and is flung up 30 feet away from the demon in a random direction and knocked prone. If a thrown target strikes an object, such as a wall or floor, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 16 Dexterity saving throw or take the same damage and be knocked prone.

If the saving throw is successful, the target takes half the bludgeoning damage and isn't flung away or knocked prone.

Lightning Strike (Recharge 6). A creature within 120 feet of the zephyr demon must make a DC 16 Dexterity saving throw or take 30 (10d6) lightning damage and be stunned until the end of its next turn.

Summon Mephits (2/Day). The zephyr demon summons 2d6 Dust Mephits, Smoke Mephits, or Steam Mephits of its choice. The mephits appear in a space within 60 feet of its summoner, are allies of its summoner, and cannot summon other mephits. They remain until the zephyr demon dismisses them as a bonus action or dies.