The Salakar, or the Vessel of Souls in elven, was an extremely powerful ancient artifact which was developed by the elves and sealed deep beneath Svartálfaheimr ages prior to the Muspel invasion of the land. The Salakar had the power to drain an individual's souls and keep it contained within itself with any other spirits it had captured. For this reason, the seemingly-immortal Isu warlord Surtr sought the device for himself, hoping to steal the "light" of Odin's son Baldr and thereby start Ragnarök.
After Odin discovered that Baldr had been taken by the Salakar in his ultimately futile attempt to rescue his son, he used it to ensnare Surtr during their final clash at the base of the main Tower in Eitri. He originally abandoned the relic with Surtr's remains, but later baited Juno into collecting it as a possible method to save Aita. Odin informed her that it would need to be emptied first, secretly trusting that her efforts would release Baldr back into the world after the oncoming cataclysm.
Due to the mysterious nature of the elves and their sacred knowledge of the soul, even the wisest among the Isu like Ivaldi were incapable of understanding the Salakar's true workings. To the best of the Isu's knowledge, the Salakar was able to contain the souls of individuals, and once captured, it was believed there was no way to leave it, though Odin expressed a small hope that Baldr's spirit could escape if Juno emptied the vessel.
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