Albel Nox, Commander of the Dragon Brigade (AU)

A man, wandering in the mountains alone, came to a pass guarded by a huge warrior. Having no weapons, or indeed any other possessions but for an empty rucksack, he decided that the best course would be to frighten the ruffian away. He called out, "What are you afraid of?" The other haughtily answered that he feared nothing. How did the man get rid of the guard?

He said him, "I have nothing in my rucksack."

... A simple riddle, when you know the answer. An old one in Airyglyph; it's a dragon in the version I learned.