Finally, there was Critias, leader of the Thirty. How I wish Plutarch had written his biography! He might fairly be compared to Sulla. Both overthrew a democracy and launched a bloody killing of their rivals and potential rivals. In neither case did the government they established last for long. While Sulla appears to have wanted a narrow and tight oligarchy but actually established a dictatorship which he seems to have intended only as a short-term necessity to achieve his goal, Critias established an extremely narrow, extremely tight oligarchy but would almost certainly have preferred a dictatorship if he could have gotten away with it.
Perhaps Plutarch declined to write a biography of Critias because he did not want to write about a straight-up villain. Yet he included both Sulla and Lysander, who certainly rated as villains in most people's eye. Maybe he thought that their military achievements made Sulla and Lysander somewhat more than simple villains. But Critias, too, had his own achievements that mark him as more than a simple villain, although those achievements were more intellectual than military. My main source on him is the International Encyclopedia of Philosophy, which calls him a "philosopher, rhetorician, poet, historian, and political leader" and seeks to piece together his views from the few surviving fragments of his work, linked here, and from what other ancient sources have said about him. I have accessed at least a few of those sources by Perseus and Google.
Critias came from one of Athens' most distinguished families, a descendent of Solon's brother and of cousin of Plato.* His father's name was Callaeschrus. He was a pupil of Socrates, indeed, when Socrates was accused of "corrupting the youth of Athens," it was almost certainly Critias who was at the top of their list,** although Critias would appear to have been only about ten years younger than Socrates.
Critias appears as a character in several of Plato's dialogues. In Protagoras he speaks only briefly, urging the principles to continue their discussion and the listeners not to take sides. He does not appear in the Republic, but in Timaeus, he announces that he was present during the Republic and that the ideal society Plato outlines bears a remarkable resemblance to one that his great-grandfather Dropides told him the Egyptians described to Solon as the most ancient form of government in Athens. The dialogue then goes on to feature Timaeus giving an account of the creation of the world, with a promise to get back to Critias and his ideal society in the next dialogue. Indeed, the next dialogue, aptly named Critias, is supposed to be Critias' account of this society, but most of it is lost; it breaks off in the middle of his account of the wealth and splendor of the lost society of Atlantis and does not get to either Athens or any purported social order. (It should go without saying that this is pure fiction, intended solely to give Plato's imaginary ideal society in the Republic some real-world credibility).
Finally, there is the Charmides, named for Critias' cousin Charmides, who would later serve under the Thirty as one of the Ten in charge of the port. This dialogue is probably the one that features Critias the most. We meet Charmides, a youth of such stunning beauty that all present, even Socrates, are stricken with lust for him, and when Socrates catches a glimpse beneath his garments, he nearly loses his self-control. Everyone assures Socrates that Charmides is as temperate as he is beautiful, so Socrates re-asserts his self-control by beginning a philosophical dialogue on the nature of temperance. He quickly demolishes the young and inexperienced Charmides' definitions (specifically, that temperance is minding one's own business, an idea he got from Critias). The more sophisticated Critias therefore steps up and gives Socrates quite a run for his his money. They soon wander off into a discussion of whether there can be knowledge of knowledge itself, and I can only say that the ensuing dialogue may be fascinating to professional philosophers, but to an Enlightened Layperson like me, it is mostly incomprehensible. Presumably, though, it can be taken to mean that Plato had great regard for Critias as a philosopher and saw him as one of the few who could actually go toe-to-toe with the master and not be completely demolished.
As for his intellectual achievements, the Encyclopedia comments that few other classical Greek writers present such a breadth of literary output. He wrote poetry, plays, homilies and aphorisms, speeches, and accounts of various city-states in poetry and prose. The article believes that he was too much of a generalist to be much of an original thinker, but that he made a significant contribution to philosophy in being the first to clearly articulate a distinction between perception by the senses and understanding by the mind. It speculates that he may have been the first to write philosophical dialogues (a form many of Socrates' pupils did) and that he may have been the first to write a politeia (combined history, civics and anthropology) of individuals cities. Certainly he wrote politeias of Sparta, Athens, Thessaly, and probably others. He was highly admiring of Sparta, although the few fragments of his work that remain address mostly drinking customs and drinking cups. As for Athens, he was presumably critical of the democracy. At least one Athenian politeia that his critical of the democracy survives. Once attributed to Xenophon, it gives signs of having been written at a time when Xenophon was a mere child.*** One possibility is that it was by Critias. If so, it may be his sole surviving work. He also wrote poetry and plays and speeches. His speeches are described as not high-flown or ornate, but but concise, vigorous, and appropriate to the occasion. (The speeches Xenophon attributes to Critias are said to mirror his style well).
It his his moral and social philosophy that are disturbing. These main surviving example of these are the fragment of his play, Sisyphus. It dismisses gods as a mere invention to make people believe that their most secret deeds will still be observed. Of course, since this is a quote from a play, and one about a legendarily impious man, it is fair to ask whether the playwright endorses this view. The Encyclopedia believes that it is supported by his other fragments, as well as by the conduct in his life. It concludes from this that he believed a wise and learned man could rise above law, religion and morality.
As for Critias' life before joining the Thirty, aside from his intellectual attainments, our knowledge is sparse. Thucydides never so much as mentions him, nor does Xenophon before his entry as one of the Thirty, which presumably means he never had a military command of any importance. A Critias is mentioned as a cousin of Andocides, and as being wrongfully arrested in connection with the mutilation of the herms, but released on Andocides' testimony. All modern historians I have read assume that this was the Critias. If so, the episode must have strengthened his disdain for the democracy.
His actions during the oligarchy of the 400 are unknown. A Callaeschrus is tantalizingly named among the 400 and may have been Critias' father, but we have no further information on the subject. But there are two strong clues that he was not among the 400. First, once they were overthrown it was Critias who made to motion to recall Alcibiades (and commemorated it in a poem). The fact that he was still in Athens after the 400 were overthrown shows that he was not one of the hard line faction. Second, the Thirty later passed a law denying citizenship to anyone who acted against the 400 or demolished their fort. This excluded Theramenes but presumably not Critias and would be incompatible with his belonging to the moderate faction. Under the restored democracy he posthumously prosecuted Phrynicus, one of the leaders of the 400, for treason. (Usual caveat about no way to be sure this was the Critias).
Some time between the overthrow of the 400 (411 BC) and the trial of the generals (406 BC), Critias fled to Thessaly under unknown circumstances, probably related to the disgrace of Alcibiades. There, Xenophon says, he "got among men who put lawlessness before justice," and there that he went wrong. Thermanes added the detail that he spent his time there "establishing a democracy in Thessaly along with Prometheus, and arming the serfs against their masters." Under the oligarchy of the Thirty, this was a serious accusation. No details are known.
This then, is what we know or can conjecture about Critias before his return. I will conclude with a quote from the Cartoon History of the Universe (page 340) in which Socrates tells Critias, "The ideal state, Critias, my boy, is run by philosophers and defended by brainwashed boys trained only to obey." There is no evidence whatever that Socrates held this view, but it is not too unfair a caricature of Plato's Republic. So, when Critias, whose intellectual attainments we have seen, returned backed by a Spartan garrison, the author has Socrates say, "This could be it!" After all, in Critias they appeared to have their philosopher king, and in the Spartan garrison they definitely had their brainwashed boys trained only to obey.
Next we will see how that turned out.
*This information appears to be from Plato who does, indeed, identify Critias as the great-grandson of Dropides, a "relative" (not necessarily brother) of Solon, but most people suspect that either he was skipping generations, or that the Critias in question was the Critias' grandfather.
**Alcibiades was a close second.
***It strongly links democracy to naval power and discussed the immense advantage a naval power has in mobility, since a prolonged overland march is impractical. This dates it to some time before 424, when Brasidas successfully undertook just such a march. Xenophon is estimated to have been about five years old at the time.