A piru is a minor evil spirit or demon in Finnish mythology. 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.