Nisser, the Danish, or rather Scandinavian, small and cute gods of protection. As far as I know, the nisse can form when an old farmer dies and is buried on their farm. The nisse will then henceforth protect the farm and it's inhabitants.

Now, while considered fun and cute in modern times, the gårdsnisser (contrary to the more feral and much more dangerous skovnisser/forest nisser) are very gullable with a strong sense of guilt, and they are hotheaded and intractable/stubborn to an unreasonable degree. They will protect you and give good luck if treated well, but will be intolerable if they feel inconvenienced or not properly cared for, to the point where they might directly or indirectly kill everyone on the farm out of spite.

Sometimes they cause atrocities because of misunderstandings, and when finding out that it was a misunderstanding on their part will cause harm to others to make up to their own farm, such as stealing cattle from neighboring farms to make up for the cows they killed earlier.

Now, the possibly worst thing you can do is to try and force a nisse to show itself to you. The small creatures accept indirect gifts, but don't like direct contact and have a cursing bite, and will most definitely bite if cornered, such as by a nosy and persistent child. This curse makes one fall sick, loosing health and strenght until one perishes in a matter of months/years. As far as I'm aware, there is no cure for the curse, and regarding the nissers tendency to be fooled or act before thinking, well....

Having a nisse on your farm can be a great blessing, but one is also constantly in danger of having their whole family killed over a minor misunderstanding or mood swing. So it is heavily recommended not to acquire any farms where a nisse might reside.