Ophidians

Ophidians were what the human inhabitants of the Serpent Isle called themselves after the arrival of the three serpents (the Chaos Serpent, the Order Serpent and the Great Earth Serpent). After being separated from the rest of Sosaria, they developed their own religion of Chaos, Order and Balance. In that time the the huge underground cities of Spinebreaker, Skullcrusher and Furnace were erected.

The culture of the Ophidians was destroyed when they killed each other in the War of Imbalance. In the end the followers of Chaos and Balance were slaughtered, and the remaining followers of Order left the Serpent Isle by the Wall of Lights looking for another world to live in. Even though they are gone, the ruins and writings of the Ophidians can be found strewn all over the Serpent Isle.

In Ultima X
Creatures called "Ophidians" were to be included in the cancelled Ultima X. These Ophidians, however, were not humans, but snake-like, swamp-dwelling warriors. See Ultima X Ophidian for details. 

Trivia

 * In Ultima Underworld II, the Avatar recovers the Blackrock Serpent of Order from a goblin, who says that he got it from a man from Pagan. This may imply that followers of Order ended up in the world of Pagan after leaving the Serpent Isle. However, there isn't any sign or mention of them in Ultima VIII, which may be due to the plot cuts that the game had before its release.