The Casket of Ancient Winters is an immensely powerful artifact of elemental power, it has been most notably used by the Frost giant and Malekith the Accursed. An Asgardian relic and the greatest weapon of Malekith the Accursed. When opened, it can reproduce the infinite icy cold of Niflheim. Later entrusted to the care of Edwin Jarvis, butler to the Avengers, by Thor. Causes its holder to grow younger gradually.
In Thor, the Casket of Ancient Winters was a treasured weapon of the Frost Giants until it was taken by Odin after he triumphed over the Frost Giants. Odin kept the Casket in his vault so no one could ever abuse its power. When Loki learned of his true heritage, he offered the Frost Giants entry into to Asgard, the return of the Casket and the chance to murder Odin in exchange for fostering a new peace treaty between Asgard and Jotunheim.
The Avengers: Earth's Mightiest Heroes
In Season One, Episode 20 of "The Avengers: Earth's Mightiest Heroes", the Casket of Ancient Winters makes an appearance. The Casket is shown to be hidden deep in an icy mountain range. Malekith the Accursed gains possession of the Casket and unleashes the infinite cold of Niflheim on the Earth, causing the temperature to drop worldwide and snow monsters accompanied by dark elves begin to appear in New York City. Some of the Avengers begin to battle these foes, while Thor, Iron Man and Black Panther try to find a way to stop Malekith. The trio find Malekith and begin to engage him in battle but find themselves overwhelmed due to Malekith being in control of the Casket. Iron Man and Thor discover that by combining the small amount of iron in Tony's suit and Thor's magic they can harm Malekith enough to incapacitate him and Thor finally is able to close the Casket thus ending the icy atmosphere that covered the Earth and the monsters that were spawned are sent back.