“It is the duty of every honest person to write the truth about Stalin,” Anton Antonov-Ovseyenko, a Soviet historian and dissident, wrote in the preface of his seminal book “The Time of Stalin: Portrait of a Tyranny,” published illegally in 1981.

A survivor of the gulag whose parents died in Stalin’s purges, Mr. Antonov-Ovseyenko spent a lifetime in almost fanatical devotion to that duty, working until his death on Tuesday in Moscow at 93 to expose the darkest truths of the Soviet era.

His books cracked through the shell of Soviet censorship that surrounded much of the Stalin-era brutality, offering readers at home and in the West a vivid portrait of tyranny and violence....