The Buckingham Palace vault is a hidden crypt under the palace gardens that holds the Shroud of Eden, the Piece of Eden both the Assassins and Starrick are chasing in Syndicate. You finally reach it during A Night to Remember.
Edward Kenway first hid the Shroud in the Tower of London. Evie follows clues from the Kenway Mansion to a key hidden atop St. Paul's, and papers on Prince Albert show the Shroud was moved here.
At the Queen's ball Starrick steals the key from Evie and opens the case. After a long fight, the twins strip the Shroud from him, kill him and leave it where it lay.
The Assassins and Abstergo both find the crypt through the twins' memories. Rebecca is hurt, the Templars take the Shroud, and Shaun kills Isabelle Ardant.
A Night to RememberA Night to Remember is the final main memory: the twins enter the Queen's ball at Buckingham Palace, Evie retrieves the vault plans from the White Drawing Room while Jacob rescues captive Royal Guards on the roofs, and both face Crawford Starrick in the vault below.Buckingham Palace
Buckingham Palace is Queen Victoria's London home in Westminster and the setting for the climax of Syndicate's story, the Queen's ball. It is also where the Queen hands out the Operation side memories, and it has a viewpoint.
Palace of Westminster
The Palace of Westminster, home of Parliament, is in Westminster beside the Clock Tower. It has a viewpoint, Jacob assassinates the Earl of Cardigan here in Motion to Impeach, and the twins later stop a Templar bomb plot inside.
St James' Park
St James' Park is London's oldest royal park, just east of Buckingham Palace in Westminster. In Triple Theft it is where Jacob and Roth go after Starrick's head of security, Benjamin Raffles.
Victoria station
Victoria station is a Westminster railway terminus south of Buckingham Palace. In Double Trouble, Jacob comes here after Catherine Gladstone, who is under guard, while gathering invitations to the Queen's ball.