The Axumillary Orb was a weapon.
The Master called it the most concentrated explosive device in history. The device could fit into a standard clothes pocket, but had enough power to pulverise an entire planet. Only one was ever made.
The Time Lords considered it so dangerous that it was placed on the Consolidator with the Sild. After the ship arrived at Praxilion, the Orb was obtained by the natives and the Red Queen made it part of her sceptre. She gave it to the Third Doctor, who threatened the Sild that he would detonate it to stop their taking over the universe. (PROSE: Harvest of Time)