The Septimal Key is an Old Kingdom artifact summoned in a complicated ritual in the Old Abbey on the monk homeland of Hook Coast. The Septimal Key is a monumentally important piece of the puzzle to unlock the Sword of Aeons. It is used to activate the Focus Sites located in various areas of Albion which would summon the Sword of Aeons.
In the time of the Hero of Oakvale, Jack of Blades was after the Sword and used Maze to do the ritual whilst Jack rounded up the Hero's family (the Hero was captured for a moment but Theresa freed him with her last ounce of Will). Maze finished the ritual, and Jack got the key. He then proceeded to activate the Focus Sites.
In Fable III, it is stated that Jack used the weapon Jack's Hammer to activate the Focus Sites across Albion. It is then quite possible that this hammer was the Septimal Key. Or, it could simply be another historical inaccuracy.