The Collaborative International Dictionary of English v.0.48:

Erebus \Er"e*bus\, n. [L., fr. Gr. ?.]
   1. (Greek Myth.) A place of nether darkness, being the gloomy
      space through which the souls passed to Hades. See
      Milton's "Paradise Lost," Book II., line 883.
      [1913 Webster]

   2. (Greek Myth.) The son of Chaos and brother of Nox, who
      dwelt in Erebus.
      [1913 Webster]

            To the infernal deep, with Erebus and tortures vile.
                                                  --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Erebus
    n 1: (Greek mythology) Greek god of darkness who dwelt in the
         underworld; son of Chaos; brother of Nox; father of Aether
         and Day

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "Erebus":
   Aides, Aidoneus, Cerberus, Charon, Cora, Despoina, Dis, Dis pater,
   Egyptian darkness, Hades, Hel, Kore, Loki, Minos, Orcus, Osiris,
   Persephassa, Persephone, Pluto, Proserpina, Proserpine,
   Rhadamanthus, Satan, blackness, dark, darkness, darkness visible,
   dead of night, intense darkness, lightlessness, moonlessness,
   night, obscure, obscure darkness, obscurity, pitch-darkness,
   pitchy darkness, starlessness, sunlessness, swarthiness,
   tenebrosity, tenebrousness, the palpable obscure, total darkness,
   velvet darkness