The crystal skull is a kind of magic jar for the ghost of Friedrich and the mace exudes an aura of evil to anyone or thing that can detect it. Friedrich can and will only emerge from the skull to attack if the mace is broken (in a critical fumble or otherwise). If this happens, the crystal will cease to glow and lose its power.
Long ago, a necromancer bound Friedrich to the weapon. It’s magic power derives from this and is very different in the hands of someone like its creator. Every time the magic power of the mace is invoked, Friedrich experiences pain beyond imagining - a fact he will be eager to share with a party and seek revenge for should the opportunity ever arise. Though the mace can be repaired if broken, if Friedrich is not somehow re-bound to the crystal its only value is monetary.
Friedrich the Ghost: AC 0 , HD 8, HP 35, Attacks: Touch (d8 cold damage), Saving Throw: 8, Special: Touch attacks also cause aging of 10-40 years (Save allowed); immune to spell attacks cast from the material plane; can only be hit by silver or magic weapons. Friedrich is completely insane and will attack a human male before others in the belief that he’s responsible for originally binding him.
Accompanying image licensed under the Creative Commons Attribution Share Alike 3.0 Unported license. Based a prior work by Steaphan Greene and public domain image brought to my attention by Telecanter.