To unlock this achievement you must kill at least 20 enemies by stealth. You will require the stealth level 5 perk 'Stealth Kill' before you can kill in this way. You will then need to acquire a dagger and keep it in your inventory at all times. Although you cannot use the dagger freely, having one in your inventory allows you to execute stealth kills against bandits or Cumans. It does not work on guards or civilians.