Hades Helmet (ハデスの兜 Hades no Helmet) refers to the Gift that grants the wearer Invisibility. It's taken from the Greek myth.It is one of the two mass copied Gifts that Perseus made for its members. With the original loaned to the right hand man of Laius Perseus to take down the No Names in the Gift Game 3 for Leticia Draculea.
Copies do not have the crest seen in the picture and can only turn the person invisible. They are still able to be heard and smelled.
The original is able to hide the user’s heat signature, smell and even sound and turn the person invisible. However, as mentioned in the light novel, its flaw is that it only makes the wearer transparent, meaning that the person is still there as a solid object. Because of this Yō was able to emit ultrasonic waves using her Gift that allowed her to locate the wearer's whereabouts, and tell Izayoi where he was resulting in their defeat.
Hades Helmet is from the Greek mythology. It was made by the Cyclops who give it to Hades (Plouton) during the Titanomachy. The others gifts being the thunder for Zeus, and the Trident for Poseidon. The helmet can put the wearer invisible. It is commonly said to be a helmet, however some describe it as a cap.
Famous bearer were Hades, Athena, Hermes, and Perseus who notably used it to flee the Gorgons after killing Medusa.
The Helmet also have different name like Helm of Darkness or Kunée (lit. dog's skin). The latter is used in some languages (French and Italian mostly) to name the Helmet.