A statue depicts Theseus slaying the Minotaur

Theseus is a popular hero in the mythology of ancient Greece. He was the son of Aegus, king of Athens, and Aethra, daughter of the king of Troezen. In some versions of the story of Theseus, it is told that he was the son of Poseidon and Aegus; this is possible only after Aethra slept with both the king of Athens and the god of the ocean in one night. This detail of having two fathers is common in Greek mythology, and it was said to have blessed Theseus with both mortal and immortal qualities.
Theseus grew up in Troezen, the small town outside of Athens where his mother was queen. his father had departed to Athens before Theseus was born. Before he left, Aegus placed his sword and sandals under a large stone. He stated that once Theseus had reached manhood, he would roll away the stone, collect the tools underneath and present himself in Athens to be accepted as a prince.
When Theseus reached his adolescence, he was lead to the stone by his mother. He rolled it away with ease and collected the shoes and the sword. As Theseus prepared to leave for Athens, Aethra begged him to travel by sea because the roads were treacherous and infested with thieves and murderers.

Theseus and Aethra, by Laurent de La Hyre

Theseus, perhaps with a rebellious, teenage spirit, decided to travel to Athens by road, paying no heed to the warning of danger. Theseus had heard the tales of  Heraclesthe Greek hero whose heroic exploits had gained him fame at this time. Theseus was determined to prove his worth as a hero by traveling the perilous road alone.
Theseus encountered and defeated many villains on the way to Athens. He defeated a murderer named Periphetes, who was known for assaulting travelers with his iron club. Theseus conquered the monster and took his weapon as a trophy.
He then crushed a man named Procrustes, or the stretcher. This murderer would tie travelers to his iron bed. Any migrant who was shorter than the bed would be stretched to painful lengths. If they were too tall for the bed, the victim would have parts of his or her limbs chopped off by Procrustes. Theseus slew him as well and began to gain a reputation as a hero.
After overcoming the perils of the road, Theseus arrived in Athens to face new dangers.  Medeathe sorceress who had fled from Corinth after her separation with Jasonhad married Theseus’ father and feared the return of prince Theseus. She worried that with the coming of her husband’s son, her influence over the king would weaken. She convinced king Aegus to be wary of the young stranger and present him with a  cup of poison. 
However as soon as Theseus stepped forward to drink the elixir, his father recognized the sword that hung from the young man’s belt. The king slapped the cup from his son’s hand and accepted him as the rightful heir of Athens. Medea is said to have fled, in fear of the certain repercussions of her treachery.
Minotaur statue

Minotaur statue

Theseus’ reunion with his father was short lived. The Athenians were in a state of peril and great affliction. Every year, Athens was forced to send seven maidens and seven young men to the island of Crete. The youths were presented to King Minos who demanded human offerings to be fed to his monster, the Minotaur.
The Minotaur was a vicious creature with the head of a bull and the body of a man. It was said to possess great strength and feed on the flesh of humans. The Minotaur was locked in a labyrinth constructed by Daedalus, the brilliant inventor who would one day create wings of flight for himself and his son IcarusThe labyrinth was said to be so artfully contrived that anybody caught in it, would never find an escape unassisted. The victims caught in the labyrinth would wander helplessly, deeper and deeper into the maze until they were eventually caught and devoured by the Minotaur.
Theseus, hearing the tale of the Athenian tributes and the fate that awaited them, swore to defeat this monster and prove himself a worthy hero. At the outset of his journey, Theseus boarded a ship with black sails headed for the island of Crete and the Minotaur. He swore to his father that upon his triumphant return, he would fly white sails so that all of Athens would know of his victory.
When Theseus arrived at Crete with the other tributes he was brought before King Minos and his daughter AriadneThe young princess was deeply enamored with Theseus and decided to help him in his task to defeat the Minotaur. In the dead of night, Ariadne came to the prison where the Athenian tributes were being held. She freed Theseus and presented him with a sword and a ball of thread so that he might successfully navigate the labyrinth. Theseus swore that if he was successful, he would bring the princess with him on his return to Athens.
Ariadne lead Theseus to the entrance of the labyrinth where he tied one end of the thread to the door post, brandished his sword and entered to labyrinth to slay the monster. Daedalus, the creator of the labyrinth, had previously instructed Ariadne on the proper way to navigate the labyrinth. Theseus followed these instructions, heading straight down and never left or right.

Theseus Victor of the Minotaur, 18th century painting by Charles Edouard Chaise.

In the dead of night Theseus came upon the sleeping Minotaur in the heart of the Labyrinth. The beast awoke and a great battle ensued. Theseus stabbed the monster in the throat with his sword and then decapitated its head completely. Following the thread, Theseus found his way to the exit and escaped that very night with the Athenian tributes and the princess Ariadne.
What happens next is of some dispute. Some versions of the myth say that Theseus left the princess on the beaches of Crete and sailed off without her. Other, more popular, versions describe how Theseus abandoned Ariadne on the island of Naxos while she slept. It was said that the goddess Hera had appeared to the Athenian hero in a dream and commanded him to leave the young woman behind. The god Dionysus would later witness the young woman crying on the shores of Naxos and marry her.
Theseus sailed to Athens, still distraught over abandoning Ariadne. the hero was so deeply upset that he forgot to replace the black sails with white ones, as he had promised his father. The king of Athens was waiting on a cliff as the ship approached. Upon seeing the black sails he was so overcome with grief, believing that his son was dead, that he cast himself off the cliff and onto the rocks below.
Theseus would become the king of Athens, returning home as a triumphant hero. The story of his slaying of the Minotaur remains a popular story to this day. It has been retold and re imagined over the years, making an appearance in several forms of media. Most recently the battle of Theseus and the Minotaur was reinvented and retold in the 2011 film The Immortals.