The Abyss Worm is an enemy in Final Fantasy IV. It can be found in the Lunar Tunnel. Abyss Worm uses an attack called Vampire that saps the target's HP every turn. The best way to defeat it is to have Rydia or Fusoya cast Break on it to instantly KO it.
Abyss refers to a bottomless pit, to the underworld, to the deepest ocean floor, or to hell, or more loosely to any chasm or large drop.