Vättar, Swedish Folk-lore...

Theme: Myths

From Wikipedia:

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 (though it should be noted that the singular vittra and huldra, respectively, refer to a solitary and quite different being). The Norwegian vetter is used much in the same way as the Old Norse vættir, whereas the corresponding word in Swedish or Danish is väsen or væsen (being), also akin to was and were.

Many aspects of the dwarves (dvergar) in Norse mythology lived on in the Scandinavian belief in vættir. They were thought to be similar in appearance to humans, even strikingly beautiful, but smaller, often clad in grey and living underground. Therefore, they were also called de underjordiske (the subterranean ones). The cautious peasant in old Scandinavia should always warn the vættir before spilling hot water on the ground, or else grave retribution, such as disease, accidents or killed livestock, was to be expected. Vættir had their own minute cattle, from which they nevertheless got a tremendous amount of milk. They were also described as having the ability go invisible from human sight whenever they wished to, as well as transform into animals (toads being a disguise of choice). This made them hard to observe, save brief glimpses; children, however, were thought to be much more capable to see through the magic of the wights.

The tomte or nisse is a solitary vätte, living on the farmstead. He is usually benevolent and helpful, which can not be said about a mischievous illvätte. However he can cause a lot of damage if he is angry, such as killing livestock.