Fimbulwinter is a prophesied winter event in Midgard. It is a three-year long period of heavy snow and blizzards which serves as a foreboding occurrence for Ragnarok, the total annihilation of all the Nine Realms.
In the God of War Series
In the wake of Baldur's death, the heavy winter had slowly taken course, with Brok noting its presence. In the secret ending, Fimbulwinter was coming to end when Kratos and Atreus encountered Thor at their house.