What did Joseph Stalin do while in power?

Stalin introduced Five-year plans that led to rapid industrialization of the Soviet Union. At the same time he collectivized agriculture by converting individual farms into huge and often ineffective collective farms. This caused or contributed to massive famine and deaths.

He created and promoted the cult of personality (his), where state media credited him personally with every Soviet achievement (though never failures), and people thought of him as their personal protector and benefactor, thanking him profusely in speeches for his wisdom and generosity.

He caused deaths of many millions of his own people by conducting purges of real and perceived enemies of the sate. In the process, he created a massive free labor force of convicts, which in part explains his successes in industrialization.

He he strengthened the Communist Party's monopoly on power and ideology, creating a totalitarian regime.
Crimes against humanity namely genocide.