I think it's a mistake to assume devil fairies are just cutesy critters with horns. The best dark fantasy uses them to explore deeper corruption, like a beautiful flower that poisons the soil it grows in. Their allure is the point—they weaponize charm and whimsy to make the darkness feel more invasive and personal.
In older folklore, they might be tricksters, but contemporary takes, especially in indie-published stuff, often link them to themes of addiction or predatory relationships. I've read a few web serials where characters mistake a devil fairy's "bargain" for salvation, only to have their sense of self slowly dismantled. The horror isn't in a gory attack; it's in the gradual, smiling erosion of your will.
You rarely see them as mere minions. When they serve a greater demon or dark lord, it's usually with a terrifying degree of autonomy, like a lawyer twisting the letter of a contract. Their power lies in loopholes and semantic traps, which feels very modern and unsettling. They make you complicit in your own downfall.