Critias (Greek Κριτίας, 460-403 BC), was born in Athens, son of Callaeschrus, was the uncle of Plato, leading member of the Thirty Tyrants, and one of the most violent. He was an associate of Socrates, a fact that did not endear Socrates to the Athenian public. He was noted in his day for his tragedies, elegies and prose works. Some, like Sextus Empiricus, believe that Critias authored the Sisyphus fragment; others, however, attribute it to Euripides.

Critias appears as a character in Plato's dialogues Charmides and Protagoras. The Critias character in Plato's dialogues Timaeus and Critias is often identified as the son of Callaeschrus - but not by Plato; and given the old age of the Critias in these two dialogues, he must be the grandfather of the son of Callaeschrus.

Critias was a very dark person in Athenian history. After the fall of Athens to the Spartans he blacklisted many of its citizens as a leading member of the Thirty Tyrants. Most of his prisoners were executed and their wealth was confiscated. He proved to be a tormented personality with many complexes and much hatred in contrast to the Platonic figure described as the student of Socrates.
He was killed in a battle on the hill of Munychia when Thrasybulus seized the Piraeus, and managed to defeat the oligarchs.

References

• Davies, J. K. (1971). Athenian propertied families 600-300 BC. London: Oxford University Press.
  
• Rosenmeyer, Thomas G. (1949). "The family of Critias". American Journal of Philology 70: 404-410.
  

Critias is also a work by Plato, see Critias (Plato).