A piru is a term that applies to minor evil spirits or more broadly to paranormal phenomenon in the folklore of Finland.
In folklore, a piru is often featured as a nasty spirit of the forest with which a wise-aleck either wins or loses a battle of wits, giving or receiving a forfeit in return.
In many cases, poltergeist and haunting phenomenons are described as pirus. The Devil is often referred to as proper noun Piru or Pääpiru, the main piru. Piru is also a mild swearword in Finnish.