Prum not Plum, it is the name used in the YP translation. Fiona is the fictional goddess that Pallums used to worship before the gods came down to the lower world. She was based off of a group of Pallum knights who were latter deified as a goddess.

My bad about the term. But wait, a group of knights were worshiped as a goddess? How does that make sense?

The concept itself actually comes from the real world. Much of mythology is actually based off of real places, things, and people. The Cyclopean walls of Mycenae were probably named because the Greeks of their dark age couldn't comprehend that the walls had been made by humans so they assumed that Cyclop craftsmen built them. Knossos was a Minoan city that was used in the Theseus and the Minotaur myth as the Labyrinth. Gilgamesh was a real king of the real city Uruk both of which found their way into Mesopotamian mythology.

Take a group of Pallum knights called the Fianna add several centuries and a dark age, it becomes very easy for people to create a mythological figure to represent their values and deify it as their goddess.

One possibility that crossed my mind is that Fianna could have been a spirit.

... You do relize that it's Phiana, the Phallum Goddess, right? 

Fiona was not actually a god, but a group of knights the Pallum people deified as a god.