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The Kings and Princes of Jötunheimr gain immortality and a ruthlessness unmatched by any other by having their hearts consigned to the Casket of Ancient Winters. Odin, thanks to the Well of Mímir, knew such a ceremony would leave the Casket vulnerable for the taking. Odin brought the forces of Asgard to Jötunheimr on the day Loki's heart was to be sealed away, disrupting the ceremony half way through.
When Loki learns that he has only ever lived with half of a heart, he sets to finish what was begun. But events don't proceed quite as he plans...