- You may be looking for the Coronet of Rassilon.
The Crown of Rassilon was one of the many technological artifacts created by Rassilon, and was one of the Presidental Regalia of Gallifrey. It was used to access the Matrix on Gallifrey. (TV: The Invasion of Time)
The crown was used by President Borusa to access the Matrix during another attempt by Omega to return to his native universe. It was used to find ways to deal with Omega and find the correct wording for the execution warrant for the Fifth Doctor. (TV: Arc of Infinity)
Pandora manifested herself through the crown when President Romana II placed it upon her head after she declared herself Imperiatrix and took it from K9 so that he could save Leela, Darkel, Narvin, her and the chapters from bombs planted in the council chambers by Antimon. (AUDIO: Imperiatrix)