"The Casket of Ancient Winters" is the twentieth episode of The Avengers: Earth's Mightiest Heroes.
Not knowing this, Black Panther, Iron Man and Thor face Radioactive Man who wants to power himself with the arc reactor of Stark Enterprises. After defeating Radioactive Man, Thor gets into an argument with Stark about the evils of mortal science. Suddenly, it starts snowing, and Thor recognizes it's due to the magic of the Casket, which Odin hid on Earth long ago; This allows Tony to counter Thor's argument now that the damage is caused by Asgardian magic. Black Panther deduces the Casket is in Norway.
Meanwhile Hawkeye, Hulk and Wasp, who enjoyed a sunny day in Avengers Mansion, are surprised by the sudden weather change and must face ice creatures who emerge from the snow. Captain America faces some of the same creatures and gets some help from Human Torch and Thing.
Black Panther, Iron Man and Thor head to Norway and face Malekith, who reveals he intends to turn Earth into a new home for the Dark Elves. Black Panther tells the other two Avengers that in Wakanda, science and mysticism work at unison, and they must do the same. Thor supercharges Iron Man, who uses Malekith's weakness to iron against him. After the Avengers defeat Malekith and close the Casket, the defrosted Enchantress and Skurge take it from their hands and leave. Thor attempts to go to Asgard to warn the Aesir about Amora plotting something, but finds out he cannot return to Asgard.