The Abyss Worm is an enemy in Final Fantasy X, that can be bred and fought at the Monster Arena in the Calm Lands. It is unlocked when the player captures at least one of every fiend in Bikanel. The player receives x99 Shadow Gem for unlocking it.
Like the other worms (Sand Worm and Land Worm), the Abyss Worm will Swallow a party member and Regurgitate them, causing 9,000 to 9,999 damage and inflicting Delay (strong). It also uses Earthquake like Land Worms and Sand Worms, but the Abyss Worm uses Readying Quake before it does the attack (in two versions). One of them, inflicts Delay (weak).
From the start of battle Abyss Worm is immune to Sleep until he uses Earthquake. After Earthquake has been used his Sleep Resist is lowered from Immune to 80%.
Its Regurgitate attack is glitched and may end up making party members invisible until the swallowed characters have left the battleground for a summon, or the battle ends, provided a character is under the effect of Guard or Sentinel when Abyss Worm uses this ability. The glitch does not affect gameplay.
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.