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 Forest Vault was one of the Cursed Vaults, located in a forest grove of the Forbidden Forest. When tampered with, this vault unleashed a sleepwalking curse on the Hogwarts students, which happened in the 1987–1988 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. At some point during his Hogwarts years, Jacob had found the Forest Vault and left behind a portrait of a dragon and a small sweater.
In the 1986–1987 school year, the vault was tampered with by an unknown person. Jacob's sibling eventually found the vault. However, its entrance was guarded by an Acromantula, which decided to put its lair there to feed on sleepwalking students. Jacob's sibling and their friends were able to defeat the creature, and find the objects left by Jacob. It would only open when they used the Centaur colony's arrow - as one part of it was found in the Vault of Fear the year before - on it. The vault then opened for them, which stopped the sleepwalking curse.