Liekkiö or Ihtiriekko is one of the scariest mythical creatures, in my opinion (I have a fobia of dead children, damn those Japanese horror movies...). Liekkiö is a ghost of an child that was left in to the woods to die, or that was murdered and then taken to the forest. These ghost children frightens, with it's yelling and crying, the travellers bypassing the childs place of death, or grave if the child was already dead.
So Liekkiö isn't really that scary or malicious, but if I'd stumble across one of them while walking in the dark woods at night... It's just horrible, from many aspects... It's horrible, and sad that some mom's actually killed their children.
Korpiklaani has made some awesome songs about Liekkiö.
This song, for some reason, describes Liekkiö as vegancefull spirit of fire, that brings devastation and famine, to... maybe those who killed the child? Or then I have misunderstood the lyrics, since it uses some very old fashioned words that I don't know yet. So I might be wrong, maybe this isn't about the same mythological creature, after all... But I still like this song.