At least some content in this article is derived from information featured in Harry Potter: Hogwarts Mystery. Spoilers will be present within the article.
The Vault of Fear was one of the Cursed Vaults, located in a secret room that was hidden behind one of the bookshelves in the Restricted Section of the Hogwarts Library. When tampered with, this vault unleashed a great number of boggarts around the school, which happened in the 1986–1987 school year.
The Cursed Vaults were thought to have been created by the founders of Hogwarts or one of the Headteachers who went insane. Over time, they became a myth and could never be found. Many years before September 1986, the Vault of Fear was found and many boggarts were unleashed in the school. The problem was eventually solved. Jacob had found the Vault of Fear and left behind a map of the Forbidden Forest and an arrow.
In the 1986–1987 school year, the vault was tampered with by an unknown person. Jacob's sibling eventually found the vault and objects left by Jacob. It would only open when they proved they had sacrificed their magic. They fooled the vault into thinking that Jacob's wand, found in the Vault of Ice the year before, was their own and that they had sacrificed their own magic. The vault then opened for them. It was guarded by the boggarts, which in Jacob's sibling's case took the form of three Voldemorts. They defeated the three boggarts and the curse stopped.