It is an Asgardian relic, which, when opened, can reproduce the infinite icy cold of Niflheim, one of the Asgardian Nine Worlds. It was the greatest weapon of Malekith the Accursed. It was used by the fire demon Surtur to cause chaos on Earth; this would help facilitate Surtur's invasion plans. It caused snow-related weather across the planet for a time, until Thor and his allies neutralize the threat. It was later entrusted to the care of the Avengers butler, Edwin Jarvis by the Norse god of Thunder Thor.
When Iceman later came into contact with it, it affected his level of control over his powers.
An alternate universe version also affected the Iceman of the Mutant X universe, but this time, permanently. This Iceman could not touch another entity without freezing them instantly.