Etymology
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Hades 

"god of the dead in Greek mythology;" also the name of his realm, the abode of the dead spirits, 1590s, from Greek Haidēs, in Homer the name of the god of the underworld, son of Kronos and Rhea, brother of Zeus and Poseidon. His name is of unknown origin. Perhaps literally "the invisible" [Watkins], from privative prefix a- + idein "to see" (from PIE root *weid- "to see"). The name of the god was extended in later Greek writing to his kingdom, also "the grave, death." Related: Hadal (adj.), 1964; Hadean.

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Pluto (n.)

Roman god of the underworld, early 14c., from Latin Pluto, Pluton, from Greek Ploutōn "god of wealth," from ploutos "wealth, riches," probably originally "overflowing," from PIE root *pleu- "to flow." The alternative Greek name or epithet of Hades in his function as the god of wealth (precious metals and gems, coming from beneath the earth, form part of his realm). The planet (since downgraded) was discovered 1930 by U.S. astronomer Clyde W. Tombaugh; Minerva also was suggested as a name for it. The cartoon dog first appeared in Walt Disney's "Moose Hunt," released April 1931.

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*weid- 
Proto-Indo-European root meaning "to see."

It forms all or part of: advice; advise; belvedere; clairvoyant; deja vu; Druid; eidetic; eidolon; envy; evident; guide; guidon; guise; guy (n.1) "small rope, chain, wire;" Gwendolyn; Hades; history; idea; ideo-; idol; idyll; improvisation; improvise; interview; invidious; kaleidoscope; -oid; penguin; polyhistor; prevision; provide; providence; prudent; purvey; purview; review; revise; Rig Veda; story (n.1) "connected account or narration of some happening;" supervise; survey; twit; unwitting; Veda; vide; view; visa; visage; vision; visit; visor; vista; voyeur; wise (adj.) "learned, sagacious, cunning;" wise (n.) "way of proceeding, manner;" wisdom; wiseacre; wit (n.) "mental capacity;" wit (v.) "to know;" witenagemot; witting; wot.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit veda "I know;" Avestan vaeda "I know;" Greek oida, Doric woida "I know," idein "to see;" Old Irish fis "vision," find "white," i.e. "clearly seen," fiuss "knowledge;" Welsh gwyn, Gaulish vindos, Breton gwenn "white;" Gothic, Old Swedish, Old English witan "to know;" Gothic weitan "to see;" English wise, German wissen "to know;" Lithuanian vysti "to see;" Bulgarian vidya "I see;" Polish widzieć "to see," wiedzieć "to know;" Russian videt' "to see," vest' "news," Old Russian vedat' "to know."
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Tartarus (n.)
in Homer and older Greek mythology, the sunless abyss below Hades, from Greek Tartaros, of uncertain origin; "prob. a word of imitative origin, suggestive of something frightful" [Klein]. Later in Greek almost synonymous with Hades.
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Sheol (n.)
1590s, from Hebrew, literally "the underworld, Hades," of unknown origin. Used in R.V. in place of Hell in many passages.
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Cerberus 

"watch-dog guardian of Hades," late 14c., Latinized form of Greek Kerberos, which is of unknown origin, according to Klein it is perhaps cognate with Sanskrit karbarah, sabalah "spotted, speckled;" Sabalah was the name of one of the two dogs of Yama. Usually represented with three heads.

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underworld (n.)
c. 1600, "the lower world, Hades, place of departed souls," also "the earth, the world below the skies," as distinguished from heaven. Similar formation in German unterwelt, Dutch onderwereld, Danish underverden. Meaning "lower level of society" is first recorded 1890; "criminals and organized crime collectively" is attested from 1900.
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Erebus 
in Homer, etc., the place of darkness between Earth and Hades, from Latin Erebus, from Greek Erebos, which is of unknown origin, perhaps from Semitic (compare Hebrew erebh "sunset, evening"), or from PIE *regw-es- "darkness" (source also of Sanskrit rajas "the atmosphere, thick air, mist, darkness;" Gothic rikwis "darkness"). Used figuratively of darkness from 1590s.
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Lethe 

mythical river of Hades (whose water when drunk caused forgetfulness of the past), in Homer, a place of oblivion in the lower world; from Greek lēthē, literally "forgetfulness, oblivion," from PIE root *ladh- "be hidden" (see latent). Related to lēthargos "forgetful" and cognate with Latin latere "to be hidden." Also the name of a personification of oblivion, a daughter of Eris. Related: Lethean.

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Danaid (n.)

in Greek mythology, one of the 50 daughters of Danaus, king of Argos, from Greek Danaides (plural). On command of their father, all (except Hypermnestra) killed their husbands on their wedding night and consequently were condemned in Hades to draw water perpetually in bottomless buckets. Hence often in reference to endless, futile labor. Related: Danaidean.

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