Älvor

Sir Olof was out riding when he happened upon dancing fairies. They asked him to dance with them, but he refused. He was on his way to his wedding. He left and during his travels, he died from a sickness put upon him by the Älvdrottning.

— Summary of "The Balad of Herr Olof och älvorna"

Älvor, or fairies, can take different shapes depending on the area where they live or who is looking at them. They can for example look like little women in white, frogs, or even insects.
When mist banks sweep in over wet fields it's easy to mistake it as Älvor dressed in white dancing in circles. When the Älvor has disappeared from the area Älvringar, fairy rings, can be found. These are circles of either healthy grass, unhealthy grass, or mushrooms. They usually try to tempt humans who come upon them while they dance into joining them. The humans who do join them will notice when they manage to stop dancing that days or even years will have passed while they danced.
Älvor lives together in fields, mounds or bogs.
Älvor are quick to take offense and they are vengeful. They have a tendency to put sickness on people. Especially unchristened children. To cure this sickness you could grease an Älvkvarn with fat and put something valuable in it, like a coin or scrapings of gold.
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Sources

Älvor, https://sv.wikipedia.org/wiki/%C3%84lvor.
Älvor, https://www.nordiskamuseet.se/blogg/alvor.
Egerkrans, Johan: Nordiska väsen. S22-27, 2013.