In Greek mythology, Hades gave Perseus the helmet that made him invisible. The object is commonly called the Cap of Invisibility or the Helm of Hades.
Perseus needed the helmet while completing his mission against Medusa and the other Gorgons. Invisibility was especially useful because the Gorgons were dangerous opponents, and the surviving sisters pursued Perseus after Medusa was killed.
The helmet is sometimes attributed to Athena or Hermes in modern retellings because both gods assist Perseus in different ways. Traditional accounts distinguish the gifts: Hades supplied the invisible helmet, Hermes provided winged sandals, and Athena supplied equipment or guidance.