History
The Casket of Darkness was an Asgardian artifact, one of the most dangerous in Odin's Vault which contains the mystical "All-Dark"; a pitch black shapeless entity that ends life. It was used by Loki in his attempt to defeat Thor, Doctor Strange and Jane Foster.
With the power of the Eye of Agamotto and the All-Father, Loki was able control the movement of the All-Dark, he unleashed the devastating power of this artifact on the World Tree where it began to spread and consume it, resulting in the potential destruction of the universe.
All efforts to remove it in vain, as the All-Dark had been enchanted by Loki's magic. Thor was able to convince Loki to use the Eye of Agamotto to see that even he will not survive the All-Dark's advance, Loki then relinquished the Eye in denial believing it to be a lie, which returned to the possession of Doctor Strange who used it to remove the enchantment while Thor and Jane used Mjolnir's lighting to disintegrate the All-Dark.
As Loki struggled to piece together the bits of ashes, the All-Dark reformed itself around the trickster as it consumed him.[1]