An usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. In other words, one who takes the power of a country, city, or established region for oneself, without any formal or legal right to claim it as one's own.  Usurpers can rise to power in a region by often unexpected physical force, as well as through political influence and deceit.
The word originally came from the Latin word usurpare (“to seize", "to take forcefully" or "to use”). 
The Greeks had their own conception of what usurpers were, calling them tyrants.  In the ancient Greek usage, a tyrant (tyrannos/τύραννος in Greek) was an individual who rose to power via unconstitutional or illegitimate means, usually not being an heir to an existing throne.  Such individuals were perceived negatively by political philosophers such as Socrates, Plato and Aristotle.  
Usurpers often try to legitimize their position by claiming to be a descendant of a ruler that they may or may not be related to.
Plato was an ancient Greek philosopher born in Athens during the Classical period in Ancient Greece. In Athens, Plato founded the Academy, a philosophical school where he taught the philosophical doctrines that would later become known as Platonism. Plato was a pen name derived, apparently, from the nickname given to him by his wrestling coach – allegedly a reference to his physical broadness. According to Alexander of Miletus quoted by Diogenes of Sinope his actual name was Aristocles, son of Ariston, of the deme Collytus.
Pericles was a Greek politician and general during the Golden Age of Athens. He was prominent and influential in Athenian politics, particularly between the Greco-Persian Wars and the Peloponnesian War, and was acclaimed by Thucydides, a contemporary historian, as "the first citizen of Athens". Pericles turned the Delian League into an Athenian empire and led his countrymen during the first two years of the Peloponnesian War. The period during which he led Athens, roughly from 461 to 429 BC, is sometimes known as the "Age of Pericles", but the period thus denoted can include times as early as the Persian Wars or as late as the following century.
A sophist was a teacher in ancient Greece in the fifth and fourth centuries BC. Sophists specialized in one or more subject areas, such as philosophy, rhetoric, music, athletics, and mathematics. They taught arete – "virtue" or "excellence" – predominantly to young statesmen and nobility.
Thrasybulus was an Athenian general and democratic leader. In 411 BC, in the wake of an oligarchic coup at Athens, the pro-democracy sailors at Samos elected him as a general, making him a primary leader of the ultimately successful democratic resistance to the coup. As general, he was responsible for recalling the controversial nobleman Alcibiades from exile, and the two worked together extensively over the next several years. In 411 and 410, Thrasybulus was in command along with Alcibiades and others at several critical Athenian naval victories.
Alcibiades was an Athenian statesman and general. The last of the Alcmaeonidae, he played a major role in the second half of the Peloponnesian War as a strategic advisor, military commander, and politician, but subsequently fell from prominence.
Polis, plural poleis, means 'city' in Greek. In Ancient Greece, it originally referred to an administrative and religious city center as distinct from the rest of the city. Later it also came to mean the body of citizens under a city's jurisdiction. In modern historiography the term is normally used to refer to the ancient Greek city-states, such as Classical Athens and its contemporaries, and thus is often translated as 'city-state'. The poleis were not like other primordial ancient city-states such as Tyre or Sidon, which were ruled by a king or a small oligarchy; rather, they were political entities ruled by their bodies of citizens.
A tyrant, in the modern English usage of the word, is an absolute ruler who is unrestrained by law, or one who has usurped a legitimate ruler's sovereignty. Often portrayed as cruel, tyrants may defend their positions by resorting to repressive means. The original Greek term meant an absolute sovereign who came to power without constitutional right, yet the word had a neutral connotation during the Archaic and early Classical periods. However, Greek philosopher Plato saw tyrannos as a negative word, and on account of the decisive influence of philosophy on politics,
Plato deemed tyranny the "fourth and worst disorder of a state." Tyrants lack "the very faculty that is the instrument of judgment"—reason. The tyrannical man is enslaved because the best part of him (reason) is enslaved, and likewise, the tyrannical state is enslaved, because it too lacks reason and order.
The Thirty Tyrants were a pro-Spartan oligarchy installed in Athens in 404 BC, after the Athenian democracy had been defeated by Sparta in the Peloponnesian War. Upon Lysander's request, the Thirty were elected as a tyrannical government, not just as a legislative committee. Although they maintained power for only a brief eight months, their reign resulted in the killing of 5% of the Athenian population, the confiscation of citizens' property and the exile of other democratic supporters. They became known as the "Thirty Tyrants" because of their cruel and oppressive tactics. The two leading members were Critias and Theramenes.
Cleisthenes, or Clisthenes, was an ancient Athenian lawgiver credited with reforming the constitution of ancient Athens and setting it on a democratic footing in 508 BC. For these accomplishments, historians refer to him as "the father of Athenian democracy." He was a member of the aristocratic Alcmaeonid clan. He was the younger son of Megacles and Agariste making him the maternal grandson of the tyrant Cleisthenes of Sicyon. He was also credited with increasing the power of the Athenian citizens' assembly and for reducing the power of the nobility over Athenian politics.
The Alcmaeonidae or Alcmaeonids were a wealthy and powerful noble family of ancient Athens, a branch of the Neleides who claimed descent from the mythological Alcmaeon, the great-grandson of Nestor.
Politeia is an ancient Greek word used in Greek political thought, especially that of Plato and Aristotle. Derived from the word polis ("city-state"), it has a range of meanings from "the rights of citizens" to a "form of government".
Politics is a work of political philosophy by Aristotle, a 4th-century BC Greek philosopher.
A democracy is a political system, or a system of decision-making within an institution, organization, or state, in which all members have an equal share of power. Modern democracies are characterized by two capabilities of their citizens that differentiate them fundamentally from earlier forms of government: to intervene in society and have their sovereign held accountable to the international laws of other governments of their kind. Democratic government is commonly juxtaposed with oligarchic and monarchic systems, which are ruled by a minority and a sole monarch respectively.
Thucydides, son of Melesias was a prominent politician of ancient Athens and the leader for a number of years of the powerful conservative faction. While it is likely he is related to the later historian and general Thucydides, son of Olorus, the details are uncertain; maternal grandfather and grandson fits the available evidence.
Ephialtes was an ancient Athenian politician and an early leader of the democratic movement there. In the late 460s BC, he oversaw reforms that diminished the power of the Areopagus, a traditional bastion of conservatism, and which are considered by many modern historians to mark the beginning of the radical democracy for which Athens would become famous. These powers included the scrutiny and control of office holders, and the judicial functions in state trials. He introduced pay for public officeholders, reduced the property qualifications for holding a public office, and created a new definition of citizenship. Ephialtes, however, would not live to participate in this new form of government for long. In 461 BC, he was assassinated, probably at the instigation of resentful oligarchs, and the political leadership of Athens passed to his deputy, Pericles.
The Athenian coup of 411 BC was the result of a revolution that took place during the Peloponnesian War between Athens and Sparta. The coup overthrew the democratic government of ancient Athens and replaced it with a short-lived oligarchy known as the Four Hundred.
Plato was an ancient Greek philosopher, the second of the trio of ancient Greeks including Socrates and Aristotle said to have laid the philosophical foundations of Western culture.
The city of Athens during the classical period of ancient Greece was the major urban centre of the notable polis (city-state) of the same name, located in Attica, Greece, leading the Delian League in the Peloponnesian War against Sparta and the Peloponnesian League. Athenian democracy was established in 508 BC under Cleisthenes following the tyranny of Isagoras. This system remained remarkably stable, and with a few brief interruptions remained in place for 180 years, until 322 BC. The peak of Athenian hegemony was achieved in the 440s to 430s BC, known as the Age of Pericles.
Athenian democracy developed around the 6th century BC in the Greek city-state of Athens, comprising the city of Athens and the surrounding territory of Attica. Although Athens is the most famous ancient Greek democratic city-state, it was not the only one, nor was it the first; multiple other city-states adopted similar democratic constitutions before Athens. By the late 4th century BC as many as half of the over one thousand existing Greek city-states might have been democracies.
The Athenian Revolution was a revolt by the people of Athens that overthrew the ruling aristocratic oligarchy, establishing the almost century-long self-governance of Athens in the form of a participatory democracy – open to all free male citizens. It was a reaction to a broader trend of tyranny that had swept through Athens and the rest of Greece.