The Casket of Ancient Winters is a relic and weapon that once belonged to the Frost Giants of Jotunheim who used it to vanquish enemy armies and conquer enemy realms. It is capable of producing and projecting an infinite icy wind that would freeze enemies and plunge a realm into an Ice Age. It was captured by the Einherjar of Asgard and sealed in Odin's Vault before being stolen by Loki.
In 965 A.D., Laufey of Jotunheim used the power of the Casket in an attempt to enslave Earth to cause a new Ice Age, but he was defeated in battle by Odin, and the relic was taken by the Asgardians and brought to Asgard, where it was kept in Odin's Vault along with many other powerful relics.
During Thor's coronation, Frost Giants, with the aid of Loki, tried to steal the Casket, but they were vaporized by the Destroyer. When Loki discovered his origins as Laufey's son, he took the Casket for himself; once he gained the throne of Asgard, he used the power of the Casket to freeze Heimdall, who was opposing him as a ruler. He then froze the Bifrost Bridge open by touching it, seemingly accessing the power of the Casket, so that the energy would build until it destroyed Jotunheim, intending to please Odin and be named the rightful heir of Asgard. When Thor destroyed the Bridge with Mjølnir to prevent this from happening, Loki willingly fell into the black hole left behind, seemingly taking the Casket with him.
When opened, it can reproduce the infinite, icy cold of Jotunheim. The casket can produce cold ranging from a concentrated beam that can freeze individual enemies, to a large vortex of icy wind that can freeze an entire landscape. It is thought to only be held by a Frost Giant, while it inflicts freezing burns on others who so much as touch it, while the Asgardians, who took the Casket, managed to touch it without harm.
- When Loki used the Casket's powers, it revealed his true nature as a Frost Giant.
- Loki demonstrated being able to cause the Casket to appear and then disappear into thin air in a warped, swirling blur of motion when he used it to freeze Heimdall, though he has not demonstrated this since.
- In the comics, the Casket had the power to bring about "Fimbulwinter", a great winter that proceeded Ragnarok.