Barion is a walled city or hill-town located on the eastern coast of Fillory near the Darkling Woods, north of the Great Salt River. It is known for its distillation of clear alcoholic liquor out of a native grain.
In The Secret Sea, Jane Chatwin, the youngest sibling of the Chatwin Family, was forced to undergo a series of trials to prove her ability to become a Queen of Fillory and take her throne at Castle Whitespire, risking permanent banishment if she failed. The trials were absurd and seemingly impossible, but Jane was able to complete them using her intelligence.
At one point, Jane is tasked with moving the city of Barion north of the Great Salt River, which she accomplished by asking Fiona Chatwin, her older sister who was already a Queen, to decree that the directions North and South were to be switched with each other. By the events of The Wandering Dune, it's revealed that Fiona never undid the decree due to the Rabbit Captain of the Windswept complaining about having had to get all new maps and compasses.
On the second day of their journey to the Northern Marsh, Eliot Waugh wanted to travel west from the Burnt River to Umber's Tears or Barion, but Janet Pluchinsky convinced him to continue on their course, promising to go through Barion when they returned. After leaving the Marsh and heading towards Barion, Pluchinsky told Waugh the story of how she came across her axes Sorrow and Sorrow during a journey to collect taxes from a village in the Wandering Desert while Waugh was away on Earth with Quentin Coldwater.
After the Fillorians United member Bayler was thrown in the dungeon at Whitespire for attempting to assassinate Eliot Waugh, he began sending his organization messages through the dishwasher to plan an attack at Waugh's wedding to Idri, which Josh Hoberman discovered after accidentally knocking the servant out with a powerful love potion.
Instead of executing Bayler, Margo Hanson devised a plan to incapacitate the F.U. Fighters using a large batch of Hoberman's love potion, tracking the dishwasher through Barion and to their camp the edge of the Darkling Woods.