Hades (gr. Αδης - ha'-deez or Αιδης - hay'-deez) means both the ancient Greek "hell" or abode of the dead and the god of that underworld (the corresponding Roman god was Pluto). Haidou was the genitive form of the word, meaning "the house of Hades"; its nominative form, Haides, was originally a designation of the abode of the dead. (A Hebrew word, She'Ol, for the abode of the dead also meant literally "unseen.")
In Greek mythology, the god of the underworld was named "Hades" because and after he was given the unseen kingdom of that name. The three brothers (sons of the Titan Chronus) Zeus, Poseidon and Hades drew lots for realms to rule. Zeus got the sky, Poseidon got the seas, and Hades received the underworld, the unseen realm to which the dead go upon leaving the world. His kingdom was entered by crossing the river Styx, ferried across by Charon (kair'-on). The far side of the river was guarded by Cerberus, the three-headed dog defeated by Heracles (or Hercules as the Romans referred to him). Hades obtained his eventual wife, Persephone, through trickery - a story that was a late addition to Greek mythology, connected with the rise of the Mysteries.
In the Myth of Perseus, the title figure acquires a helm of invisibility that had originally belonged to Hades. The helm was made for Hades by (?), who also gave lightning to Zeus and the trident to Poseidon, to aid in the war against the Titans.
Hades was a fearsome figure to those still living; in no hurry to meet him, they were reticent to swear oaths in his name. To many, simply to say the word "Hades" was frightening. So, a euphemism was pressed into use. Since precious minerals come from under the earth (i.e., the "underworld" ruled by Hades) he was considered to have control of these as well, and was referred to as "Ploutos", Greek "wealth." This explains the name given him by the Romans, "Pluto."