Who is Andrei Yanuaryevich Vyshinsky from The Gulag Archipelago, 1918-1956 and what is their importance?

A lawyer and diplomat, Menshevik turned Bolshevik, Vyshinsky is Chief State Prosecutor during the great show trials of 1936-38, which is where Solzhenitsyn primarily shows him, formulating a useful dialectic for interrogators and prosecutors: there is no such thing as absolute truth any more. Officials must use their own intellect, Party sensitivity, moral forces, and character to render a verdict. Solzhenitsyn observes that the executioner's bullet is absolute.