Medusa's Head

Medusa's head is horribly fatal to anyone that looks directly at it, even if it is not attached to its body. It was said be absolutely horrendous and her hair is made of snakes.

History
This was originally a punishment to her from Athena after she desecrated her temple with Poseidon. Anyone who looks at her face will turn to stone, even if the head is chopped off. Perseus, a demigod son of Zeus, cut off Medusa's head as a wedding present for King Polydectes, only to find out the wedding was a trick to take Perseus's mother, Danae. A person can only look at Medusa's eyes from a reflection, otherwise they're turned into a stone statue. It is unknown what happened to it afterwards.

The Lightning Thief
Percy Jackson sends Medusa's head to his father, who eventually sends it back to him. Percy then gives it to his mother Sally Jackson so that she is able to turn her abusive husband to stone. After Sally sends the stone statue of her husband to an art collector, naming it "The Poker Player", it is hailed by mortals as "a huge step forward in super ugly neorealism".

The Lightning Thief
Percy Jackson cuts Medusa's head off as she is distracted. At Annabeth Chase's suggestion, they take it with them in case it could be used as a weapon. It is wrapped in Grover Underwood's shirt. 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. When collecting one of Persephone's Pearls, 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).

Trivia

 * So far both demigods who have cut off Medusa's head have had the same name, though different fathers. Perseus, son of Zeus, and Perseus Jackson, son of Poseidon.