Hello RobT,
"Druden" or "Truden" are evil spirits or persons (mostly women) possesed by evil spirits. These evil spirits or persons were made responsible for nightmares. To protect the sleeping persons the "trudenmesser" was punched innthe bed-head. Other names are "Drudenfeitel", were "Feitel" is local (Austrian, Bavarian or South Tyrolian) word for knife. "Neunkreuzer" means nine crosses. This is because the "Trudenmesser" the blade was often adorned with seven or nine. Also these knives often had Christian mottoes and 7 or INRI punched into the blade. I did not mention it before have two "Trudenmesser" in my collection (no fotos because I am a bad photographer), one a folding knife and one fixed just like the one of Alan62 but not that beautiful. Actually the previous owner had polished and sharpened it
Geetings, Helge