The Ard Feainn (Elder Speech for Great Sun) was a division of the 3rd Nilfgaardian Army commanded by the young Colonel Ramon Tyrconnel. Because of name, it is also possible that division is the military branch of state religion.
"‘The Ard Feainn Division,’ said Julia Abatemarco. ‘If your men didn’t mistake anything. They really had banners with silver suns on them?’ ‘Yes, Colonel,’ said the commander of the scouts, without hesitation.‘So, Ard Feainn,’ mused Pretty Kitty. "
- The Lady of the Lake