Marxism-Leninism.

Lenin was a scholar and developed his own take on Marxism, which has its own understanding of the communist society. Marx wrote very little about what a communist society would look like, but he had an understanding of history as moving towards an end: The classes will fight, over time the result of this fight will lead to them approaching each other, and at the end of this struggle we will reach a classless society. This classless society is the communist society in a traditional Marxist sense.

Lenin figured he’d make a shortcut to get there: Never mind thousands of years of class struggle, let’s just put in place a powerful ruling class imposing communism on everyone, designing a classless society from the top down. Which is a bit counter-intuitive, but the Leninist part of Marxism-Leninism basically boils down to trying to figure out what that could look like.

So then you get the Soviet Union, very much founded on the ideas of Marxism-Leninism. Today people who identify as Marxist-Leninist tend to not be the sharpest tools in the shed: Despite insisting that they have studied the texts carefully, a brief interaction with them reveals that they have never read neither Marx nor Lenin. What it boils down to, rather than anything theoretical, is either a longing for some imaginary version of the Soviet Union or a unshakable commitment to lick Putin’s ass.

The Soviet Union of course never did become a classless society, so you could argue that the greatest achievement of Marxism-Leninism was to destroy the traditional meaning of communism in a Marxist sense.