Abyss
The Abyss shard was an early incarnation of what more or less later became both the Test Center and Siege Perilous shards. It featured skill gates and an early version of the Siege Ruleset.
The Abyss shard was closed July 12, 1999 and was shortly succeeded by the Siege Perilous shard three days later. The "closing event" was cataclysmic in nature and featured hoards of demons springing up and laying waste to any and everything in their path before the shard was shutdown permanently late that night.
The word "abyss" is generally associated with hellish or infernal regions.
In the Ultima context, the Stygian Abyss was a dark, evil place from the original, solo-player Ultima games. It is also the name of the next planned expansion, Ultima Online: Stygian Abyss(which is presently over a year behind schedule).
Abyss can also refer to the Abyss Slayer weapon slayer type that is effective against demons and demon-like monsters.