Vatican Vault
The Vault is one of the "Temples" left by Those Who Came Before, located underneath the Sistine Chapel in Rome. The exact purpose of the Vault and the other Temples is still unknown. Minerva's message implies that the Vault's contain some sort of assistance or research in saving of a repeat of the Catastrophe that occurred before.
It appears that only a combination of different Pieces of Eden can open it along with specific people seem to be able to enter them. Recognized as a "prophet", Ezio Auditore da Firenze was able to enter the Vault by using the Staff in conjunction with a Piece of Eden, whereas Rodrigo Borgia was unable to do so. The floors, walls and pillars of the Vault are covered in glyphs that only appear as Ezio moves near them.
It was thought by Borgia that the Vault contained a living God. However, the Vault actually contains the sole survivor of Those Who Came Before, Minerva, who appears through a holographic device. By her own admission, she is not a God, but a member of a highly advanced race that came before humans. Minerva explains that her race had created humans, but soon faced rebellion. Glyphs on the wall confirm this, showing an Egyptian hieroglyphic with a God holding a Piece of Eden as humans charge toward him with weapons.
According to Minerva, the Temples were built by "those who knew to turn away from war," presumably those who did not fight or did not want to fight in the war between the humans and Those Who Came Before. The reason for this was an unknown catastrophe, presumably a large solar flare, that significantly damaged the Earth and killed off most of both species. Those in the Temples worked to protect the world, and Minerva tasks Desmond Miles, speaking to him through Ezio, with locating the other Temples to avert another cataclysm.
Notes
- During Minerva's speech, a glyph of step pyramids with what appears to be a Piece of Eden hovering over it appears on the wall. This could possibly mean that another Temple is located in Central America or a ziggurat in Mesopotamia or even the ancient Temple of Borobudur in Indonesia.