An Unknown dimension
The Casket of Ancient Winters was an Asgardian artifact, which contains the Fimbulwinter of Ymir; it created massive snowstorms if opened. The hand size Casket of Ancient Winter contained the fury of a thousand killing winters. It was used by Surtur In his attempt to defeat Thor. Surtur unleashed the devastating power of this artifact on the planet Earth. Needless to say this event caused mass-hysteria. Thor with the aid of Earth's heroes was able to defeat Surtur and safeguard the Casket so that it could not be misused again.
Sometime later the Casket mysteriously appeared in the possession of the Dark Elf, Jagrfelm. Before he had the chance to use it for his own purpose, he was slain by the leader of the Dark Elves Malekith the Accuser. Malekith used the Casket to wage war on Asgard. In this battle he was able to bring the Mystical Kingdom down to its knees. The power of the Casket reached the realm of Hel, causing concern of Volla and Hela. The cold was so permeating that the strongest of the trolls Ulik was overtaken by this calamity. It even incapacitated the Frost Giants themselves.
There seem to be no cure to its effects, until the Lady Sif recalled the secrete Gem of Infinite Suns. Which was concealed in Odin's personal treasures. Thor retrieved the Gem and used its mighty powers to right the wrongs caused by Malekith. Once everything was returned to its natural state, Thor sent both the Casket and the Gem into an unknown dimension. Only time will tell if we will ever see either the Casket of Ancient Winter or the Gem of Infinite Suns again.