Critias in Harpers Dictionary of Classical Antiquities
（Κριτίας). An Athenian, a disciple of Socrates and Gorgias of Leontini. He was one of the most accomplished men of his time, and was distinguished as a poet and an orator. But he is best known as the chief of the Thirty Tyrants (q.v.), in defence of whose cause against the Liberators he fell in B.C. 403. He was the author of several tragedies. Some fragments of his poems have survived, the longest being from his political elegies. He seems to have had the gift of expression, but to have written in a harsh style.
Critias in Wikipedia
Critias (Greek Κριτίας Kritias, 460-403 BC), born in Athens, son of Callaeschrus, was an uncle of Plato, and a 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.
After the fall of Athens to the Spartans, Critias, as one of the Thirty Tyrants, blacklisted many of its citizens. Most of his prisoners were executed and their wealth was confiscated. Critias was killed in a battle near Piraeus, the port of Athens, between a band of pro-democracy Athenian exiles led by Thrasybulus and members and supporters of the Thirty, aided by the Spartan garrison. In the battle, the exiles put the oligarchic forces to flight, ending the rule of the Thirty.
Critias asserted that "religion was a deliberate imposture devised by some cunning man for political ends."
Critias appears as a character in Plato's dialogues Charmides and Protagoras, and according to Diogenes Laertius, he was Plato's great-uncle (Diogenes Laertius, Lives and Opinions of Eminent Philosophers, III:1). 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 may be the grandfather of the son of Callaeschrus.