Source : Webster's Revised Unabridged Dictionary (1913)

Avernal \A*ver"nal\, Avernian \A*ver"ni*an\, a.
   Of or pertaining to Avernus, a lake of Campania, in Italy,
   famous for its poisonous vapors, which ancient writers
   fancied were so malignant as to kill birds flying over it. It
   was represented by the poets to be connected with the
   infernal regions.