From Proto-Indo-European *h₁régʷos. Cognate with Old Armenian երեկ (erek, “evening”), Sanskrit रजस् (rájas, “dimness, darkness, mist”) and Old Norse røkkr (“twilight”).
Ἔρεβος • (Érebos) (genitive Ἐρέβους) n, third declension
This inflection pattern originally had a sigma (i.e. Ἐρέβεσος), which was dropped early on in the history of Ancient Greek.
The irregular accentuation of this paradigm is explained by the fact that it is the contracted version of the above paradigm.