Avengers : Season 1, Episode 20: "The Casket of Ancient Winters"

"The Casket of Ancient Winters" - On the hottest day of the year, an ancient artifact is found…and a cold wind blows. An ancient Asgardian relic called the Casket of Ancient Winters has been opened, and the entire planet goes into a deep freeze. But as the rest of the Avengers deal with an invasion of Dark Elves and demons, Iron Man and Thor must put aside their differences to battle Malekith the Accursed.