Between the summer of 1937 and November 1938, the Stalinist regime arrested over 1.5 million people for "counterrevolutionary" and "anti-Soviet" activity and either summarily executed or exiled them to the Gulag. While we now know a great deal about the experience of victims of the Great Terror, we know almost nothing about the lower- and middle-level Narodnyi Komissariat Vnutrennikh Del (NKVD), or secret police, cadres who carried out Stalin's murderous policies. Unlike the postwar, public trials of Nazi war criminals, NKVD operatives were tried secretly. And what exactly happened in those courtrooms was unknown until now. In what has been dubbed "the purge of the purgers," almost one thousand NKVD officers were prosecuted by Soviet military courts. Scapegoated for violating Soviet law, they were charged with multiple counts of fabrication of evidence, falsification of interrogation protocols, use of torture to secure "confessions," and murder during pre-trial detention of "suspects" - and many were sentenced to execution themselves. The documentation generated by these trials, including verbatim interrogation records and written confessions signed by perpetrators; testimony by victims, witnesses, and experts; and transcripts of court sessions, provides a glimpse behind the curtains of the terror. It depicts how the terror was implemented, what happened, and who was responsible, demonstrating that orders from above worked in conjunction with a series of situational factors to shape the contours of state violence. Based on chilling and revelatory new archival documents from the Ukrainian secret police archives, illuminates the darkest recesses of Soviet repression — the interrogation room, the prison cell, and the place of execution — and sheds new light on those who carried out the Great Terror.