Air essence is crafted in the Essence Refiner. Like every other recipe, the essence refiner requires a power source linked to it, either from Black Aurem, Celestial Prism, or an Obelisk, through the use of a Crystal Wrench. The recipe requires one Arcane Ash in the center with two Tarma Roots and two feathers. Air essence can be used to craft Battlemage Boots, Slipstream Generator, and Flicker Focus : Lunar Tides Air essence is also required to summon a boss, known as the Air Guardian.
Air essence can also be dropped by chance upon an Air Guardian's death. However, you still have to craft at least one beforehand to summon it.