Medusa's Head is a spoil of war obtained by killing the Gorgon Medusa. The head can turn anyone that looks directly at it to stone, even though it isn't alive. The head is said be absolutely horrendous, notable for the snakes that make up Medusa's hair.
Medusa was once a beautiful woman who was a lover of Poseidon. When Medusa and Poseidon desecrated Athena's temple, Athena cursed Medusa and her sisters, Stheno and Euryale, to turn into monsters with snakes for hair. Medusa alone was cursed to be so ugly that anyone looking at her would turn to stone. Medusa lived on an island with her sisters, the three becoming known as the Gorgons.
Some time later, Perseus, a demigod son of Zeus, was invited to a banquet by King Polydectes, who expected everyone to bring a gift. The banquet was actually a plot of Polydectes. He knew that Perseus could not provide a proper gift. Polydectes would use the opportunity to send the boy away while he himself courted the boy's mother. When Perseus arrived, he asked Polydectes to name any gift and that he would retrieve it. Polydectes told Perseus he wanted the head of Medusa, something that countless others had tried and failed to retrieve. Perseus agreed to the challenge and left right away.
On his quest, Perseus was given advice and many gifts from the gods. Zeus himself offered Perseus a sword and the Helm of Darkness to hide him from the Gorgons. Athena provided a polished shield so Perseus could see Medusa without looking directly at her. When Perseus arrived at the lair of Medusa, he cut off her head while she slept, causing Pegasus and Chrysaor to spring from her neck. Perseus then used the Helm of Darkness to escape from the other two Gorgons before using the winged sandals he received from Hermes to return home.
On his travels home, Perseus used the head of Medusa to turn his enemies to stone. Some stories claim that he used it to turn the titan Atlas to stone. But the more commonly accepted story is that Perseus left Atlas under the sky as he knew he couldn't best Atlas. Perseus had also used it on Phineus, the original suitor of Andromeda. After Perseus saved her from the monster Cetus, she had agreed to marry him instead. When Phineus objected, he was turned to stone by the sight of Medusa's head which Perseus showed him.
Once Perseus returned, he learned of King Polydectes' unwanted advances on his mother and turned him to stone. With his mission fulfilled, Perseus gave the head to Athena as a tribute. Athena took the head and created a shield from it that was named Aegis.
In a fight with Medusa, Percy Jackson cuts off her head, killing her. He is confused why Medusa's head didn't turn to dust and Annabeth Chase explains to him that it is a spoil of war like his Minotaur's horn. He then sends her head to his father, Poseidon, who eventually sends it back to him, telling him he will have to make a choice when he finds the package.
When Percy receives the head once again, he realizes what Poseidon means and offers to use it to stop his mother's abusive husband Gabe Ugliano. She refuses, wanting to save herself so Percy leaves her the head and the choice. After Gabe demands meat loaf, Sally seems to decide to use the head as Percy describes her as seeming to consider what Gabe would look like as a garden statue. Within the next week, Sally uses Medusa's head to turn Gabe to stone and sells it as a sculpture she has made called The Poker Player. It is hailed as "a huge step forward in ugly neo-realism" and Sally gets enough money from it to start a new life. While she is begged for more statues, Sally assures Percy that she disposed of Medusa's head rather than use it again.
Percy Jackson cuts Medusa's head off while she is distracted. At Annabeth's suggestion, they take it with them in case it could be used as a weapon. It is wrapped in Grover Underwood's hoodie. When the head is not in use, they put sunglasses over the head to prevent any accidental deaths. It is seen a few times in the story, such as when collecting one of Persephone's Pearls and when Grover uses it to defeat the Hydra. He then kisses the head, before reacting in disgust. When the three of them stay at a motel, it is kept in the sink as Grover states that "he cannot pee with her watching him." The trio forgot to close the curtains, and as a consequence, a maid passing by sees it and screams before fleeing. At the end of the story, it is given to Percy's mother and kept in her refrigerator, which turns Gabe to stone when he breaks open the fridge lock (placed by Percy with a warning message not to open it).
- So far both demigods who have cut off Medusa's head have had the same name, but different fathers: Perseus, son of Zeus, and Perseus Jackson, son of Poseidon.
- A person can only look at Medusa's eyes from a reflection, otherwise they're turned into a stone statue.
- Only Medusa has the power to turn people into stone. The other Gorgons, Stheno and Euryale, only have snakes for hair.