When and by whom does democracy get introduced to Athens?

After the expulsion of the tyrants in 510 BCE, there was a tussle between the aristocratic party which dominated through the Council on the Areopagus, and a populist movement led by Cleisthenes, which the later won, giving political control to an assembly of all landowning citizens.

This lasted for a couple of decades, until the Persian invasion 480-479 BCE when the aristocrats led the resistance and held onto dominance in the ongoing warfare after the invasion was repelled. In 460 BCE Ephialtes led another democratic coup and eliminated the Council's power. The aristocracy assassinated Ephialtes for this, but his sidekick Pericles took over ad embedded the democracy.