avernal

from The Century Dictionary.

• Relating or pertaining to Avernus. See Avernian.
• Destructive of bird life: as, avernal places, Stanley, Hist. Philos. (in allusion to the ancient statement that birds attempting to fly across Lake Avernus were killed by its mephitic vapors. See etymology of Avernian).
• noun A demon or other inhabitant of the infernal regions.

from the GNU version of the Collaborative International Dictionary of English.

• adjective 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.