Robbers are hostile NPCs that can be found roaming the wilderness in bands of about five. If you encounter a band of robbers, one will approach and speak to you. He will demand you hand over your most valuable items. If you agree, they will let you go unhurt. If you refuse, you can either fight or attempt to flee.
Losing a battle against robbers is not always fatal; They may simply knock you unconscious and steal your valuables, after which you will awaken injured in an unfamiliar location.
Avoiding robbers can be troublesome as they, like other human NPCs, seem to operate with higher stat caps as well as suffering reduced encumbrance penalties compared to the player.