The vættir (Old Norse; singular vættr) are spirits in Norse mythology. The term can be used to refer to the full cosmos of supernatural beings, including the álfar (elves), dvergar (dwarves), jötnar (giants), and gods (the Æsir and Vanir). Vættir can also refer more specifically to landvættir (nature spirits), fjallvættir (mountain spirits), sjóvættir (sea spirits), skogvættir (forest spirits), vatnavættir (guardians of the specific waters), or húsvættir.
The Old Norse term véttr/vættr and its English cognate wight are descended from Proto-Germanic *wihtiz (thing, creature), from Proto-Indo-European *wekti- ("object, thing"). Vættr and wight normally refer to a supernatural "being", especially landvættr (land spirit), but can refer to any creature. The Norwegian vette is used much in the same way as the Old Norse vættr as are the corresponding Swedish cognate vätte (dialect form vätter - Old Swedish vætter) and the Danish vætte. A related form in the Slavic languages can be seen in Old Church Slavonic вєшть, (veštĭ), meaning thing, matter, or subject.
Landvættir (nature spirits) are chthonic guardians of specific grounds, such as wild places or farms. When Vikings approached land, they reportedly removed their carved dragon heads from the bows of their longships so as not to frighten and thus provoke the landvættir to attack and thereby incur bad luck from them. Icelandic culture continues to celebrate the supernatural protection over the island and four landvættr can still be seen in the Icelandic coat-of-arms: a troll-bull, troll-eagle, dragon, and handsome giant. The troll-animals are actually jötunn who shape shifted into the form (and mentality) of an animal and such animals are supernaturally strong.
Húsvættir is a collective term for keepers of the household, like the English brownie, or the Nordic tomte (also referred to as Nisse). The tomte or nisse is a solitary vätte, living on the farmstead. He is usually benevolent and helpful, which can not be said about an innately mischievous illvätte. However, a nisse can cause a lot of damage if he is displeased or angry, including killing of livestock or causing serious accidents.
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Scandinavian folklore features a class of beings similar to the Old Norse landvættir. They are known by many names, although the most common are vättar in southern Sweden (singular: vätte), vittra in northern Sweden, and huldrefolk in Norway (although the singular vittra and huldra, respectively, refer to a solitary and quite different being).
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During the 19th century, Peter Christen Asbjørnsen and Jørgen Moe compiled Norwegian folk tales. These stories often reflected the animistic folk belief that preserved earlier elements derived from the Viking Age.
You are supposed to leave out a bowl of milk or porridge for your nissar (one or several living on your property). If you don't then they will play pranks on you, such as turning all your milk sour. Source: oral Swedish folklore.
They're everywhere, and are probably known under different names, like garden gnomes. there's a reason it's called Northern European folklore and not just Norwegian folklore
Well, my English mum used to tell me the same story, that you should leave out a saucer of milk. She just called those creatures fairies though (i.e. she used it as a generic term). As @mr_pigeonwizard mentions, brownie might be the more appropriate name. In Sweden I have always been told by many, many people that the nissar are not to be trifled with. They tell me this tongue-in-cheek, but still very firmly. So if I were you I would leave out a bit of milk or a saucer of creamy porridge and perhaps good things will happen!
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Scandinavian folklore features a class of beings similar to the Old Norse landvættir. They are known by many names, although the most common are vättar in southern Sweden (singular: vätte), vittra in northern Sweden, and huldrefolk in Norway (although the singular vittra and huldra, respectively, refer to a solitary and quite different being).
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During the 19th century, Peter Christen Asbjørnsen and Jørgen Moe compiled Norwegian folk tales. These stories often reflected the animistic folk belief that preserved earlier elements derived from the Viking Age.
And do be kind to those, they are helping you, and protecting the property