The Helm of Darkness

The Cyclopes made three weapons for the three brothers when the Titan war began: the thunderbolt for Zeus, the trident for Poseidon, and for Hades a helm that renders the wearer invisible. It is the only one of the three that is not a weapon at all.

Hades wore it in the war itself. In Apollodorus's account he goes invisible into the Titans' camp and destroys their weapons before the fighting starts, which decides a great deal before anyone lands a blow. It is the earliest special-operations story in Western literature.

He lent it out more than his brothers lent theirs. Perseus wore it to escape Medusa's surviving sisters, who could not follow what they could not see. Athena wore it in the Iliad so that Ares would not notice her helping Diomedes, and Hermes wore it against a giant. For a god famous for keeping to himself, Hades is unexpectedly generous with his one great possession.

The name in Greek plays on his own. Hades is often derived from a-ides, 'the unseen', so the god of the unseen owns the instrument of unseeing, and his name and his helmet mean the same thing. Whether the etymology is real or a poet's pun, the Greeks clearly enjoyed it.

It is also why he is so rarely depicted in action. The other Olympians are shown fighting; Hades is shown enthroned. A god whose signature item removes him from view is difficult to put in a painting.