Beware the Nice Ones/Myth, Legend and Folklore

Norse Mythology's Vanir were a race of fertility gods, who spent their days making the world flourish with plants and children... Oh, and they almost destroyed the Aesir.
Nymphs were usually kind and flirtatious, but piss one off and you're likely to be drowned and gutted.
Most people seem to think of Demeter as peaceful and calm, the goddess of the harvest and growth. One of the lesser known myths shows us just how awesome she can be: a King cut down her favorite tree. So she got one of her handmaidens to fetch Hunger, and sicced her on the King, until he ate everything in his city, spent and sold everything he had, even his own mother, and then ate himself.
- Not to mention the whole " If I don't get my child back, I can and will kill everything on this planet" thing.