The Vault, more precisely known as Odin's Vault, also known as Odin's Treasure Room or the Weapon's Vault, was a room in the Royal Palace of Asgard, which held a number of relics that Odin had gained as a result of his victories over the course of his life.
Odin captured, stole, purchased and did whatever he could to take all the relics and weapons that could bring about Ragnarok (the Asgardian verosion of the end of times), and he locked them away in a vault in the base of his palace, so anything he thought could bring the universe down is now under lock and key.
Home to the incredibly powerful Casket of Ancient Winters, the Weapon's Vault has always been a prime target for those seeking powerful weapons of destruction. During Thor's crowning ceremony, the Vault is attacked by three Frost Giants attempting to steal the casket. However Odin sensed the Frost Giants in the Vault and activated the Vault's deadliest automaton device the Destroyer to dispatch the the Frost Giants.
The Tesseract was placed in the vault following the Battle of New York. During the Second Dark Elf Conflict, Odin commanded the Einherjar to protect the Weapons' Vault. In the end after the Dark Elves were defeated, their weapon, the Aether, was taken by the Asgardians. However, Sif and Volstagg decided to take it to the Collector for safekeeping, since it was considered unwise to keep two Infinity Stones so close together.
After killing Surtur, Thor placed the fire demon's crown in the vault for safekeeping. Hela entered the vault during her Siege of Asgard, observing several of the relics kept there. She used some of the Eternal Flame to resurrect her Berserkers and her pet Asgardian wolf, Fenris. After realizing that Ragnarök was the only way to defeat Hela, Thor sent Loki into the vault to retrieve Surtur's crown and place it on Eternal Flame. This resurrected Surtur and unleashed Ragnarök, resulting in Hela's death, Surtur's death again, and the destruction of Asgard.
- According to Craig Kyle, the items in the Vault are items Odin believes could cause Ragnarök. Coincidentally, the Eternal Flame, when in contact with the Crown of Surtur, was eventually used by Loki to cause Ragnarök.