The Blade of Bastet is the Mistress' legendary sword. In order to make their own Blade, the dwellers need to find a historic sword and pick up a swing analyzer from the Fabricator.
Creating the Blade of Bastet requires a bit of legwork. First, the player needs a historic blade - in this case, Grant's Saber located at the Whitespring Presidential Cottage. The case with it can be opened by looting the damaged Protectron in the adjacent room for the Presidential Cottage Password, then open the case with the terminal.
Afterwards, it's a matter of returning to the Riverside Manor and using the fabricator to obtain a swing analyzer, then combining the two at any weapons workbench.
Finally, the player simply has to eliminate six different enemies with the saber (each enemy must be of a different type). Note that most of the time, the script will register it as a kill if the player hits the enemy several times with the blade and then kills the target through other means. The quest log will update whenever a qualified strike is made. The fabricator will then allow the player to craft the Blade of Bastet.