The Seventh Key is paramount over all other Keys, and grants sovereign control over the The House, although it's specific demesne are the Incomparable Gardens. It was held by Lord Sunday for over 10,000 years before it was claimed by Lord Arthur. The Seventh Key appears as a small, golden key on a chain around Lord Sunday's Neck. After Lord Arthur claims this Key, as well as the other six, the Fifth Key slides onto the chain of its own volition.
The powers of the Seventh Key are numerous, but most are unknown. Although it must be noted that any powers that the Keys have will have greatly increased effects/power when used in conjunction with the Seventh Key. The known powers are:
- Locking and Unlocking
- Opening and Closing
- Manipulation on a mass scale (such as changes in the weather)
- Accessing the Improbable Stair
- Sorcerous Supression
- Hold Back/Banish Nothing
The Seventh Key also holds the greatest amount of power, being capable of supressing both the Fifth and Sixth Keys and The Compleat Atlas. However, this was done to a disoriented Lord Arthur while in the Incomparable Gardens, which is the Seventh Key's demesne. It is unknown how difficult such an act would be in the rest of the House or in the Secondary Realms.
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.