Smersh
Smersh (short for Smert' Shpionam (Смерть Шпионам), or "Death to Spies") was formed during the Great Patriotic War, to secure the rear of the active Red Army, on the front to arrest "traitors, deserters, spies, and criminal elements".It was also used to maintain military discipline in the Red Army by means of zagradotryads, or barrier forces, that were supposed to shoot down the retreating Soviet troops. It was also used extensively after the war as well, especially in Eastern Europe. In all Soviet controlled territory it was used to conduct Stalinist witch hunts, and to purge the new communist parties, so that they would become nothing more than obedient puppet states to be ruled from Moscow.
Smersh was also used to punish those with in the NKVD itself; it was allowed to investigate whom ever it wished within the NKVD structure, department and directorate heads were not immune from it, and if it found even the slight bit of evidence that they were somehow involved in whatever plot it was that week, they would be arrested, and tortured by Smersh, forced to sign fake confessions, put on a show trial, and either sent to the camps or shot. Smersh would also often be sent out to find and kill defectors, double agents, etc.
One of its most notorious moments was when they slaughtered many Polish officers in the Katyn massacre, an atrocity that was later blamed on the Germans. Smersh was also used by INO (the NKVD's later KGB FCD, First Chief Directorate, responsible for foreign intelligence operations outside of the USSR) to hunt down "enemies of the people" outside of Soviet territory.
Eventually Smersh's activites were slowed down, but they never really ceased. Its name may have changed as the NKVD changed, but its duties always remained the same.
SMERSH (in capitalised letters) was featured in Ian Fleming's early James Bond novels as 007's nemisis. Though Fleming's SMERSH was supposed to be modelled on the real organisation the novels had SMERSH as a massive Soviet counterespionage operation aimed at sending operatives abroad to subvert the west with an additional goal of killing western spies, particularly Bond. Villain's in the Bond novels who were SMERSH agents include Le Chiffre in Casino Royale; Colonel General Grubozaboyschikov, Colonel Rosa Klebb and Donovan Grant in From Russia with Love, Auric Goldfinger in Goldfinger, Mr. Big in Live and Let Die. In later books, as well as in the James Bond movies Bond's archenemy became S.P.E.C.T.R.E Film versions of books in which SMERSH was featured substituted S.P.E.C.T.R.E as the villain.