Wikipedia
Nacht und Nebel ( German for "Night and Fog") was a directive from Adolf Hitler on 7 December 1941 targeting political activists and resistance "helpers" that was originally intended to winnow out "anyone endangering German security" (die deutsche Sicherheit gefährden) throughout Nazi Germany's occupied territories. Its name was a direct reference to a Tarnhelm spell, from Wagner's Das Rheingold.
Three months later Armed Forces High Command Feldmarschall Wilhelm Keitel expanded it to include all persons in occupied countries who had been taken into custody; if they were still alive eight days later, they were to be handed over to the Gestapo. The decree was meant to intimidate local populations into submission, by denying friends and families of seized persons any knowledge of their whereabouts or their fate. The prisoners were secretly transported to Germany, and vanished without a trace. In 1945, the abandoned Sicherheitsdienst (SD) records were found to include merely names and the initials "NN" (Nacht und Nebel); even the sites of graves were unrecorded. The Nazis even coined a new term for those who "vanished" in accordance with this decree; they were vernebelt - "transformed into mist". To this day, it is not known how many thousands of people disappeared as a result of this order.
The International Military Tribunal at Nuremberg held that the disappearances committed as part of the Nacht und Nebel program were war crimes which violated both the Hague Conventions and customary international law.