The Tremor Mace is a weapon belonging to the Iron King with the power to shake the earth. It can face off against other powerful weapons such as Monkey Khan's Atom-Bat and Amy Rose's Piko Piko Hammer.
Although appearing to be magical in nature, it is possible the Tremor Mace is a technological weapon made by Dr. Ivo Robotnik, similar to the Atom-Bat. Seeing as the Iron King was not seen with it in his earlier years, it might have been part of the supplies provided by Robotnik Prime later on in his career. It could possibly related to the Atomic Mace Ixis Naugus created before leaving for the Zone of Silence. (StH: #202)