Coronet of Rassilon

The Crown of Rassilon was used to access The Matrix on Gallifrey. (DW: The Invasion of Time)

The Crown was also used by President Borusa to access The Matrix, durring another attempt by Omega to return to this universe. It was used to find the ways to deal with Omega and to find the correct wording for the execution warrant for The Doctor. (DW:Arc of Infinity)