Unlike some prior editions, fey are not really the cutesy woodland spirits (although there are some for sure). Hags are fey, for example. Dark fey are a thing, and I suggest checking out Tome of Beasts (3PP) for great new evil/dark fey creatures.
My next adventure involves going into a fey forest. I won't go into details (at least one of my players periodically reads the forums), but it won't be cute. They already know about the grugarch, which are neutral xenophobic elves that generally attack/kill outsiders on sight. They previously met centaurs on the outskirts of the forest, warning them that the fey in the deep parts of the forest are not to be trifled with. I have a handful of encounters ready that will teach them to respect the fey, if they don't already. Oh, and those are the "good" fey...
My next adventure involves going into a fey forest. I won't go into details (at least one of my players periodically reads the forums), but it won't be cute. They already know about the grugarch, which are neutral xenophobic elves that generally attack/kill outsiders on sight. They previously met centaurs on the outskirts of the forest, warning them that the fey in the deep parts of the forest are not to be trifled with. I have a handful of encounters ready that will teach them to respect the fey, if they don't already. Oh, and those are the "good" fey...