Stifling Stillness

You make all air in the target area unnaturally still and unyielding, creating a shimmering haze. The stagnant, heavy air becomes an area of difficult terrain. Creatures in the area that breathe air and aren't holding their breath must spend a single action on their turn straining to breathe the stagnant air; once they do, they still mostly breathe their own exhaled air, taking 3d6 poison damage (basic Fortitude save) and becoming Fatigued.