The Bolsheviks storm the Winter Palace overthrowing Kerensky’s provisional government (no, the Bolsheviks did not overthrow the Czar, that was Kerensky), bringing about the nascent Soviet Union that would be the lurking shadow on world politics for the next 74 years with influence still seen today.
The train set in motion that day brought us:
The Red Terror, under Lenin. Up to 1.5 million killed.
The Holodomor under Stalin, up to 12 million killed.
The Great Purge, also under Stalin, another 200 to 600 thousand killed (almost trivial by comparison–but this included a lot of people like experienced military commanders which would come back to haunt them later).
The German-Soviet non aggression pact of 1939, giving Germany a period of peace to the East so he could focus his forces on the conquest of France to the west, thus helping encourage the start of World War II. Yes, WWII was pretty much inevitable by this point, but this may well have hastened the start, lengthened the war, and led to more death and destruction.
The Soviet invasion and subjugation of the Baltic States.
Millions of unnecessary deaths in World War II (those experienced commanders purged up above? Yeah. Those. Throwing masses of bodies at the enemy is no substitute for competent, experienced leadership. You might win in the end–as they admittedly did–but only at a far higher cost than otherwise.)
The subjugation and oppression of Eastern Europe under the Warsaw Pact.
The communist revolution in China, including the “Cultural Revolution” and “Great Leap Forward” (what an ironic name) that lead to the deaths of over a hundred million people.
Communist revolutions in Cuba and Central America, leading to yet more death, destruction, and oppression.
Communist revolutions in Southeast Asia, leading to the Khmer Rouge and the killing fields.
“Socialism” being imposed in Venezuela leading to widespread hunger and misery. (And at this point we’re only hitting select examples, the rot has spread so widely.)
All that, from the results of November 5 (Gregorian Calendar) 1917, making this arguably the blackest day in all of history.