Fimbulwinter (大いなる冬(フィンブルヴェド) Finburuwedo) is the Ten-year war started in order to gain OOParts (Magical gems that could turn certain people into magi)
After scientist discovered the rare gems known at OOParts they began experimenting with them in order to test the limits of the abilities they could give to people: Magi. Scientists decided to keep the existence of OOParts secret from the world from fear of war, but as research prolonged many the scientists thought that others were holding up on OOParts and began to distrust one-another. Eventually some scientists left "Volsupa" (The organization that was created to test the limits of magi) and joined other corporations. Wanting to continue thier research they began to fight over OOParts and used "Magi" as their weapons. The war between magi was eventually named "Fimbulwinter".
The war lasted ten years, involving alot of magi but only several of them are mentioned. Sosei Yoshino (Siegfried/Odin), Ichigo Sagara (Yggdrasil), SAKURA (Brynhildr), and Rouge Flame (The Mage of Misfortune). Rouge Flame appeared during the end of the war where he destroyed chapter after chapter of Volsupa headquarters and destroyed the machine that could generate Eye Space (A dimension created to limit the damage the war could do to the real world) all so he could set everything on fire with his flames. Sosei and SAKURA fought Rouge Flame while Ichigo fled the island with their child who was Reiji. After Rouge Flame kills SAKURA Sosei's powers as a magi awaken and he is destroyed by Sosei who becaome Odin. Rouge Flames's death is what brought about the end of the "Fimbulwinter War" as stated by Ichigo Sagara.