The Fourth Key was the key originally wielded by Sir Thursday for 10,000 years, until Arthur claimed the Key in Sir Thursday. It grants control over the Great Maze (as well as making the wielder the Commander of the Glorious Army of the Architect).
The Fourth Key takes the form of a Marshal's Baton, white and glowing green and wrapped in golden laurel leaves. When the Wielder wishes, it can take on the form of a sword; A broadsword when used by Sir Thursday and a rapier when used by Arthur. It is implied that the Key can take on the form of any weapon the wielder wishes.
- Shedding Light
- Vocal Amplification
- Instantly Destroys Victims (such as Nithlings) through physical contact
- Energy Emission (in it's full power)
- Commanding Obediance
As of Lady Friday it is said that all Keys can kill anything the wielder wishes, be it Nithling, Denizen, Superior Denizen or Mortal. It is beleived that any being not of those categories, such as The Architect, The Old One and their three children (Lord Sunday , The Mariner, and The Piper) would be unphased by any such attempts. Towards the end of the series, it is thought Lord Arthur would be immune as well. It is unknown if the Trustees were immune while wielding their Keys.