Short answers, pulled from the story.
Marxism, Leninism emerged as a distinct ideological framework shortly after Vladimir Lenin died on the 21st of January 1924. Joseph Stalin and his allies synthesized classical Marxism with Leninist praxis to create this new doctrinal system within the Communist Party of the Soviet Union.
Stalin launched the first five-year plan between 1928 and 1932 to achieve rapid industrialization in coal, iron, steel, electricity, and petroleum sectors while forcing agricultural collectivization. The program reached 23.6% collectivization within two years and achieved 98.0% collectivization thirteen years later despite causing the Soviet famine of 1930, 1933 that killed millions of people.
The Soviet Union installed native Marxist, Leninist governments across Eastern Europe following the Yalta Agreement between 4 and the 11th of February 1945. Communist parties took power in Poland, Hungary, Romania, Bulgaria, Czechoslovakia, and East Germany during this period.
Nikita Khrushchev denounced Stalin at the 20th Congress of the Communist Party of the Soviet Union on the 25th of February 1956, which China viewed as ideological betrayal. The Sino-Soviet split began in 1956 and escalated into full diplomatic rupture by 1966 when China pursued détente with the United States while challenging Soviet leadership globally.
Stéphane Courtois authored The Black Book of Communism which estimated deaths ranging from 93 to 100 million people under communist rule. These figures describe Communist democide, genocide, and Red Holocaust as direct consequences of Marxist, Leninist ideology including events like the Great Purge and Cambodia's Killing Fields massacres.