The most chilling take on an aerial villain's descent I've ever read is the webnovel 'Heaven's Defiant Throne.' The antagonist starts as a literal divine being, one of the so-called Celestial Choir, their power woven from light and the harmony of the spheres. The fall isn't a plummet; it's a slow, corrosive dissonance. You see it in the prose—descriptions of their flight growing labored, feathers molting not into darkness, but into a brittle, translucent crystal that shatters in the wind. Their grace curdles into a rigid, crystalline obsession with purity, and they start viewing the very mortals they were meant to guide as blights on creation's score. The tragedy is they believe they're performing a final, beautiful act of cleansing, right up until the moment their own song silences them mid-air, and they simply stop flying, falling like a statue. It’s less about becoming evil and more about becoming so perfectly, terribly 'good' that they destroy themselves from the inside out.
I find stories about skybound villains particularly compelling because the physical descent mirrors the psychological one so viscerally. In the anime 'Tales of Aeolia,' the Wind General Lythia doesn't lose her power to fly; she loses the will to land. Her villainy stems from a growing detachment, a literal and metaphorical altitude sickness. She ends up orchestrating wars from her floating citadel not out of malice, but a numb, clinical fascination with the patterns armies make on the ground, like an ant farm. Her final scene, where the protagonist doesn't defeat her in combat but coaxes her down to feel the grass under her feet, only for her to realize she no longer remembers the sensation, hits harder than any epic clash. It's that slow erosion of connection that makes the fall from grace feel earned, and terribly sad.