Vault of the Dark One

The Vault of the Dark One is an Enchanted Forest location on the ABC show Once Upon a Time. It makes it's debut in the Season 3 episode, "Quiet Minds".

The Vault is owned by the current Dark One, and has many rooms that hold his or her possessions, as well as various spells, ingredients, and artifacts that the Dark One can use for magical purposes. It also houses the Darkness, the substance that makes the Dark One what they are.

Before the First Curse
At some point after becoming the Dark One, Nimue constructs the Vault for her own personal usage.

The Vault is located in the ground, as a circular seal. A key is used to open the vault. When Rumplestiltskin became the Dark One, he was granted a key from the vault that allowed him to gain access to it.

If the current Dark One dies without being killed by the dagger, they can be resurrected at the vault in exchange for the life of another person.

Elsa briefly resides in the Vault, after Rumplestiltskin steals the urn she was imprisoned inside of. Rumple (unaware that Elsa was inside) took the urn until Ingrid gave him the Sorcerer's hat. Since she gave the hat to the Apprentice, the urn, and Elsa remained in the vault.

King Arthur discovers the location of the vault in his search to find both halves of Excalibur. Lancelot and Guinevere use a magic gauntlet to lead them to the location of the vault. After entering, Guinevere saves Lancelot from the darkness. They soon discover the dagger sitting on a table in one of the rooms in the vault. Rumplestiltskin appears, wanting the gauntlet. He makes a deal with Guinevere to give him the gauntlet in exchange for enchanted sand that would give the illusion that Excalibur would be whole.

Before the Second Curse
After Rumplestilstkin kills himself and his father with the the dagger, no new Dark One is created, since he used the dagger on himself. After everyone returns to the Enchanted Forest, Belle and Neal find a way to bring him back from the dead. Unbeknownst to them, the Wicked Witch of the West, Zelena, has her own plans for the Dark One. She uses Lumiere to trick Neal and Belle into leading her to the vault. Belle realizes that Lumiere is lying to them, and he admits to doing Zelena's bidding. Belle warns Neal about opening the vault, but he does so anyway, taking out the key. After the vault is opened, the key brands his hand with a triangular shaped tattoo. His life is taken in exchange for Rumplestiltskin's resurrection. Unable to accept that his son is dying, Rumple absorbs Neal into his own body, becoming one person before Neal can die.

During the Second Curse
When the Second Curse is cast, Rumple's memories are erased, and as a result does not remember absorbing Neal into his own body. After being imprisoned by Zelena, Rumple escapes, and Neal eventually takes control of their shared body. After discovering the burnt triangle symbol, Belle researches it and discovers that it is connected to the Dark One's Vault. Because Neal used the vault to resurrect the Dark One, he was branded and must pay with his life. Neal asks Emma to separate him from his father's body so that his father can recall the identity of the Wicked Witch.

After the Second Curse
During the alternate timeline, Emma and Hook are pulled into the past by Zelena's time portal and eventually become trapped in the Vault, along with Maid Marian (who is really Zelena in disguise). The urn that Elsa is trapped in catches Hook's eye, and he pulls it out of the cabinet it was stored in. When Emma regains her magic to re-open the time portal and return to the present, the urn is also pulled through.

Before the Third Curse
After the darkness is released from Rumplestilskin, it wanders Storybrooke, trying to latch itself onto another host. To save everyone, Emma leaps toward the darkness, succumbing to it completely, becoming the new Dark One. Back in the Enchanted Forest, she arises from the Vault, in a large cloak. The darkness manifests itself into Rumplestilskin and serves as her guide until she fully embraces her powers.

Later after Hook is near death, Emma tethers his life to Excalibur, resulting in him becoming another Dark One. Hook is then seen rising from the Vault, also covered in a cloak, and led by the same voices that guided Emma.

Trivia

 * While the Darkness' main host is the Dark One, some of it also resides in the vault itself.