Calithiss was, as mentioned in Ultima VII Part Two, the last Hierophant of Order, in the time of the Ophidians on the Serpent Isle. She was called Calithiss the Steadfast before the War of Imbalance, but after Ssithnos was murdered she was known as Calithiss the Avenger, blaming the followers of Chaos for the murder.
However, from all evidence gathered, it becomes clear that she herself had led her people against Ssithnos, and afterwards blamed Chaos in an attempt to gain full power for Order over the Serpent Isle.