The Decadent are a supposed nation that was planned to be in the game, but abandoned before the official release.
"I don't know a lot about these people. They live in a fortified city which controls a straight on an important sea route. For hundreds of years they have lived from the tolls they raise there. I hear they have become decadent."
One of promotional beta screenshots refers to what became the Dji Cantos as "decadent". Coupled with the fact that there is an unused class in the game with the same name, there might have been a very different storyline originally planned. The "old Harriet" might have been a member of this group.