|“||The Labyrinth is treacherous. There is only one thing that saw me through: the love of a mortal girl. The string was only part of the answer. It was the princess who guided me.||”|
When Aegeus found out Aethra was pregnant, he returned to Athens. He met up with Medea, who had fled Corinth after she had killed Jason's wife. Some stories say she killed her children with Jason, but she denied it and said the townspeople killed them in The Lost Hero.
Theseus on his way to Athens encountered and defeated the giant Procrustes who lure travelers on to a bed and stretched or shortened his victims to the perfect size for his two beds. Theseus defeated him by trapping him in his own bed, although it is not said whether he straightened or shortened him. He also defeated: Periphetes (the clubber); Sinis (the bender of pines); Crommyonian Sow (a big pig type monster); Sciron (pushed his victims from cliffs); Cercyon (the wrestler). He killed all of them doing the same thing they did to their victims.
When he arrived at Athens, Theseus did not introduce himself immediately, and his father was suspicious of the stranger. However, Medea recognized him and worried that Theseus would be given the crown instead of her son Medus. She challenged him to capture the Cretan Bull (that was once wrestled by Hercules himself). Theseus returned to Athens victorious, so Medea subsequently planned to poison him. At the last second, the king recognized the sandals, shield and sword that Theseus was wearing as the ones he had left under a boulder for his son. He promptly knocked the poisoned wine out of Theseus' hands, and Medea fled.
The Minotaur and Ariadne
On the third occasion, Theseus volunteered to slay the monster. He took the place of one of the youths and set off with a black sail, promising to his stepfather, Aegeus, that if successful he would return with a white sail. Like the others, Theseus was stripped of his weapons when they sailed. On his arrival in Crete, Ariadne, King Minos' daughter, fell in love with Theseus and, on the advice of Daedalus, gave him a ball of thread. This was so he could find his way out of the Labyrinth. That night, Ariadne escorted Theseus to the Labyrinth, and Theseus promised that if he returned from the Labyrinth he would take Ariadne with him. As soon as Theseus entered the Labyrinth, he tied one end of the ball of string to the door post and brandished his sword which he had kept hidden from the guards inside his tunic. Theseus followed Daedalus' instructions given to Ariadne; go forwards, always down and never left or right. Theseus came to the heart of the Labyrinth and also upon the sleeping Minotaur. The beast awoke and a tremendous fight then occurred. Theseus overpowered the Minotaur with his strength and stabbed the beast in the throat with his sword (according to one scholium, Theseus strangled it on Pindar's Fifth Nemean Ode).
After decapitating the beast, Theseus used the string to escape the Labyrinth and managed to escape with all of the young Athenians and Ariadne as well as her younger sister Phaedra. On the return journey Theseus abandoned Ariadne on the island of Naxos, where she was eventually found by the god Dionysus. Ariadne then cursed Theseus to forget to change the black sail to white. Seeing a black sail, Theseus' stepfather Aegeus committed suicide by throwing himself into the sea (hence named Aegean). Theseus and the other Athenian youths returned to Athens safely, with Theseus inheriting the crown. He later claims in The Battle of the Labyrinth that he very much wanted to bring back his stepfather from the Underworld, but was unable to do so.
Theseus would later defeat wild centaur tribes and fight the Amazons. At one point, he also welcomed the wandering Oedipus into his kingdom.
In his latter years, however, Theseus fell into bad company with Pirithous (King of the Lapiths), and became far more reckless and villainous than ever before. He slew his own son, Hippolytos (due to the goddess Aphrodite, who sought to punish the latter for his aromantic asexuality); became a tyrannical king and, seeking to marry a daughter of Zeus, kidnapped a 13-year-old Helen of Sparta (this was long before Paris), deciding to hold onto her until she was old enough to marry him. Pirithous also wanted a beautiful daughter of Zeus as a wife, but he chose a much more unattainable one - Persephone herself. Leaving Helen with Theseus' mother, Aethra, Theseus and Pirithous traveled to the Underworld. An enraged Hades, however, called the Furies to punish both heroes and subsequently had them stuck to the Underworld rock they were sitting on, unable to stand up. While Theseus was eventually freed by Hercules a few months later (during the completion of his eleventh labor), Pirithous stayed there for eternity, since he had committed too great a crime for wanting the wife of one of the Big Three as his own bride.
Even after freedom, Theseus eventually grew unpopular, and perished miserably in exile.
Dionysus stops Percy Jackson and Blackjack while they were following Thalia Grace, Grover Underwood, Zoë Nightshade and Bianca di Angelo. Dionysus tells Percy that you can never trust heroes because of the account that Theseus 'dumped' Ariadne on the island Naxos, betraying her. Because she was beautiful, Dionysus healed her broken heart and married her.
Theseus's soul appeared while Nico di Angelo was trying to summon Bianca's soul. Percy thought that Theseus had been a 'middle-aged, buff guy' until he saw his ghost, which surprised him. He looked about the same age as Percy which at that time was 14. The ghost states that Aegeas was his stepfather.
Paul suggests, "Like Theseus. He was supposed to raise white sails when he came home to Athens," to which Nico replies, "Except he forgot, and his stepfather jumped off the palace roof in despair. But other than that, it was a great idea."
When Nico summons Theseus's soul in The Battle of the Labyrinth, it takes the form of a 14-year-old boy in Greek armour , with long curly dark hair and Poseidon's sea-green eyes, with a seashell-shaped clasp on his cloak. According to Percy, his voice sounded "like dry, crumpling paper", with his eyes "as lifeless as glass."
- ADHD: Like most demigods, Theseus possessed inborn supernatural battle reflexes and senses that he used to analyze the fighting style of his opponents. In Percy Jackson's Greek Heroes, Percy describes Theseus as the super-caffinated kid and the deficit demigod. Indicating that Theseus is extremely ADHD, even for a demigod.
- High Intelligence : Theseus used his wits and cunning to defeat opponents who transcend his physical strength. Percy describes Theseus as "a really smart dude" and as "a genius" when it comes to some things, especially battles.
- Hydrokinesis: In Percy Jackson's Greek Heroes, it is mentioned that Theseus once went underwater without problems. In mythology, Minos threw his ring into the sea and asked Theseus to retrieve it to prove that he is a son of Poseidon.
- Superhuman Strength: Being a demigod, Theseus was stronger than the average mortal. Theseus was physically strong enough to wrestle with and overpower foes as mighty as Procrustes and the Minotaur. He was able to easily carry Periphetes's club, that was covered with about 20 sheets of bronze. He had also held the buff king, Cercyon over his head, and dropped him on his knee killing him instantly. Theseus had even managed to slung the Marathonian Bull on his shoulders, which Percy has stated, had weighed 500 pounds.
- Fighting Skills: Theseus was an extremely skilled warrior, even inventing the combat art of pankration with Hercules. He was also skilled in hand to hand combat, as he was able to beat Cercyon with little effort at all. He was such a skilled warrior, that he slaughtered the first and second army of the Pallantides all by himself.
- Percy, another son of Poseidon, also slayed The Minotaur.
- Theseus, like Percy, inherited their father's green eyes.
- Theseus was mentioned in The House of Hades, when Hazel meets Sciron.
- In The Lightning Thief, Percy Jackson also fought Procrustes and killed him the same way.
- In Ancient Athens, where citizens valued thought and intelligence far above physical strength, Theseus was believed to be the greatest Ancient Greek hero instead of Hercules .
- Although Theseus is a son of Poseidon, Athena's nemesis, he ironically inherited rule of Athens (Athena's city) from his stepfather, Aegeus. Today, his legacy remains as he is remembered as Athens's greatest king.
- Theseus and Pirithous were the only mortals that ever attempted to capture Persephone from Hades.
- He was imprisoned in the Underworld and was later freed by Hercules around the same time as Jason met Medea. But Theseus encountered Medea early on in his life, and she had fled to Athens after giving up on Jason. This is currently an unsolved error.
- Percy, another son of Poseidon, also defeated Procrustes by trapping him on his own bed.