Piru, from Finnish mythology, is considered to be a variety of things. Traditionally, the word has meant anything from a lesser devil or evil to Satan. The word has its base in the word perkele, which is tied to the old Slavic thunder god, Peru. While usually meaning the Devil himself, the word is also used for trolls, ogres, hiisi's (Finnish form of demon), and a variety of elves and gnomes that are evil or have characteristics of lesser evils.
In Finnish mythology and shamanism, pirus (both demons and devils) are roaming spirits that actively pursue their prey and the potential victims, unlike bound spirits, such as ghosts. Striking with little or no warning, they can cause illness, insanity, change the victim's personality, or even possess the victim. (Interesting side note: the state of the person being possessed is known as riivattu. The base word is riivio, which means gremlin, so the connotation is that the person being possessed has a gremlin controlling their actions.) The piru can also possess, haunt and live in buildings themselves, as opposed to normal ghosts.
Shamans and priests may exorcise the piru out of the person, which sometimes cures the illness, but it usually takes years for the victim's psyche to recover. Folklore mentions a multitude of different spells and methods used to expel such unwanted presences, ranging from using salt to a full-scale exorcism, and sometimes burning down the building.
Due to their roving personalities, pirus have been said to crash occasions, such as weddings or baptisms. Sometimes, the piru kidnapped children that were not being watched, taking them as their own. In these occasions, the child also became an invisible roaming spirit, only able to eat and drink accursed food. For instance, the piru might have knocked a jar of milk over, after which, if the maid cursed, the piru's child would have a few moments to drink what milk had spilled. If the maid said a blessing, the child would have been punished and left without sustenance.
During the early Christian era, the piru was seen as a corrective spirit that replaced the wrongs. If a household master cheated the workers out of pay, he would eventually end up in the piru's clutches and driven to insanity. But it didn't end there, the piru would also tell the workers where the master's money and goods were hidden.
Interesting tidbit: Don't fall asleep in a sauna overnight. The piru would become upset enough to kill you.
Interesting Link: http://www.paranet.fi/paradocs/keskustelua/tikkala1993.html
Sorry, I couldn't get hold of a picture today! Have you read or watched something with a Piru in it? If so, what was it? Also, is there a mythological creature you're itching to read about? Let me know!