When Lenin died in 1924, he final testament was hushed up. Why? Because he had left the 'Party' to Trotsky and Zinioviev, and told them Stalin was not to be trusted. But by now Stalin was General Secretary of the Communist Party of the Soviet Union and all the Bureaucracy/Apparatchiks were indebted to him for their positions. With the cunning of a weasel he was able to play the Left (Trotsky) against the Right (Zinoviev and Kamenev) and then eliminate the them both. He then eliminated those in the Center and the Old Bolsheviks (members of the party prior to October 1917).
He then systematically eliminated anyone who might become a danger to him or raise a hand against him. The purpose of the Great Terror was to 'cower' everyone. When your afraid to talk to your own children for fear of exposure, your not thinking about overthrowing the government, your hoping not to be noticed. Stalin then was able to 'create' an autocracy where all great and wonderful things emanated from the Leader (him). Stalin was able to dictate the 'history' of the CPSU, the Revolution and Soviet Russia, where he was all knowing and powerful. (That he had might have been an agent of the Tsar's Okhrana, had been a minor player in the Revolution, and his bumbling during the Polish-Soviet War in 1920 cost the loss of Lvov and Warsaw was easily passed over.)
For anyone who wants to know the extent of the "Purge" and what went on during the three Great Trials, this is the answer to all your questions. Conquest does a fabulous job of explaining who was, what was what, and what happened to whom. The dialogue from the trials is alone worth reading the book for. In one case a traitor was accused of meeting foreign spies at a hotel that had been torn down ten years before, and another had a man flying to a city whose airport had been closed for six months during the winter when he flew in.
Well worth the time and effort to read (500+ of densely packed pages).
Zeev BM Halevi