Yesterday I killed an Irrlicht. First I refused to do this, but no chance, if you want to finish the main quest, you'll have to kill a Will o' the wisp or in german - Irrlicht. Bad, bad game. But ok, at least I've slayed dozens of ogres before that. :)
I think of irrlicht when I wake up in the middle of the night to feed the fire - we heat with a woodstove. Often the wood I've split has glow in the dark fungus growing on it. Makes loading the firebox a bit easier without turning on the light.