The Powers of the Fifth Key are numerous, so not all powers are known. The known powers of the Fifth Key are;
- Holding Back/Banishing Nothing
- Energy Emission
- In Superior Saturday, Lord Arthur causes one of Lord Sunday's Nithling insects to invert on itself multiple times, shrinking as it did so, until it became nonexistent. It is currently unknown exactly how this ability works.
- It can allow the wielder to travel to any reflective surface in the whole universe. They enter through a silver doorway.
- It is able to drain 'experiences' from mortals, Lady Friday then absorbed these experiences, enabling her to experience mortal emotions for a short time.
- It can also return experiences if they have only been recently drained.
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 believed 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 unfazed 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 whilst wielding their Keys.