Asgard isn't just home to powerful beings like Thor, Odin and Loki, it's also home to several powerful artifacts. A land of mystery and fables, Asgard has many fantastical items, such as the Destroyer Armor and the mischief-weaving Norn Stones. One such artifact is the incredibly destructive Casket of Ancient Winters, which contains the power to envelop whole realms in eternal winter. In both the Marvel Comics Universe and the Marvel Cinematic Universe, the Casket of Ancient Winters is usually kept under guard in Asgard, though its contents were not created there. The casket actually holds the Fimbulwinter of the great and terrible frost giant, Ymir.
As a result, the fury of Ymir's Fimbulwinter allows the user to unleash the power of a thousand deadly winters when opened. Due to the destructive nature of the artifact, it is a prime tool for use by dastardly individuals. While the casket is often under close guard, like many objects of power in the Marvel universe, they often find themselves in the hands of villains. One such event began in 1984 during Walt Simonson's run on Thor. Thor #350-353 saw a storyline known as "The Surtur Saga" play out. This saga involved Surtur waging war on Asgard and their allies, unleashing the Casket of Ancient Winters on Earth.
As panic gripped the Earth, Thor had to gather together a powerful alliance of Earth's heroes, including the Avengers and the Fantastic Four. To bolster their ranks in the battle against Surtur, Thor also called upon Loki, Beta Ray Bill and the All-Father himself, Odin. They would be needed, as Surtur and his wielding of the casket made the fiery villain a Ragnarok-level threat. The heroes prevailed and the Casket of Ancient Winters was once again placed under lock and key to prevent further misuse. This wouldn't last long as the casket would be acquired by the Dark Elf, Jagrfelm, who was then slain by the Dark Elf king, Malekith the Accursed.
Malekith, now in possession of the casket, decided he would try his hand at subjugating the Nine Realms himself. The Dark Elf unleashed its power on Asgard itself, causing the great and terrible hoarfrost within to freeze the realm and seep into others. When all seemed to be lost and all avenues were explored for a countermeasure, Lady Sif brought news to Thor of an artifact capable of reversing the winter effects of the casket. Sif reminded Thor of the tale of the Gem of Infinite Suns, which the heroes found hidden in Odin's personal possessions. When he had freed the realms using the gem, Thor hid the gem and the Casket of Ancient Winters in another dimension.
More recently, during War of the Realms, Malekith once again tried to ravage Asgard and the realms surrounding it. This time, he gave the Casket of Ancient Winters to Laufey, leader of the Frost Giants. He used the artifact to besiege New York, the stronghold to many of Earth's Heroes. The X-Men and Avengers stood valiantly, but it would be Loki who would stop Laufey and retrieve the casket. He did so by cutting his way out of Laufey's stomach with Heimdall's sword after it was thought the Frost Giant Leader had eaten the God of Mischief. It is still believed to be in his possession.
In the Marvel Cinematic Universe, the Casket of Ancient Winters is locked inside Odin's vault and has been there since Asgard defeated the Frost Giants long ago. Loki stole it and used it to try and destroy Jotunheim by freezing the Bifrost, along with Heimdall. Thor managed to stop his brother's plan from coming to fruition by shattering the bridge and Loki fell into the void below. The casket was returned to Odin's vault but was believed to have been destroyed when Surtur destroyed Asgard. Quite fitting that the one who brought about its end in the MCU is the being who unleashed it first in the comics.