Abyss Worm is an enemy in Final Fantasy X that can be bred and fought at the Monster Arena in the Calm Lands by capturing at least one of every fiend in Bikanel. The player receives 99 Shadow Gems for unlocking it. It shares the same model as the Land Worm as a towering worm with a gaping mouth.
The Abyss Worm will attack three times before preparing to use Earthquake, which damages the party and inflicts Delay (weak). If Yuna summons Valefor, the Abyss Worm will use a different version of Earthquake and the attack will miss. Otherwise, it will only use normal attacks against aeons. The Abyss Worm is also initially immune to Sleep, but after it uses Earthquake, its Sleep resistance falls to 80%. It will temporarily become immune again while it prepares to use Earthquake.
After being targeted five times by characters not afflicted with Poison or Zombie or using Doublecast or Self-Destruct, the Abyss Worm will Swallow a party member, temporarily removing them from battle. It will wait until it is targeted twice more to Regurgitate them, causing 10,000 damage and inflicting Delay (strong). It can still attack aeons in this state, however. Being targeted by aeons or while preparing to use Earthquake won't count towards Swallow and Regurgitate. Hellfire and Thor's Hammer have alternate animations against the Abyss Worm.
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. The glitch triggers when a character is under the effect of Guard or Sentinel when Abyss Worm regurgitates. This does not affect gameplay.
This battle should be easy if the player is well leveled up and has the Celestial Weapons. Summoning Valefor before the Abyss Worm uses Quake will negate the damage. Regurgitate's damage can be halved with Protect.
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.