The Avengers: Earth's Mightiest Heroes Wiki
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The Seal of Bor's Sons is an ancient powerful seal casted on Muspelheim and its ruler Surtur to prevent him and his Fire Demons from causing destruction to all inhabitants of the Nine Realms.

However, when the Norn Stones were destroyed, the seal weakens and then was destroyed thus freeing Surtur and Fire Demons to run free to cause death and destruction.

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