I could tell you where it is, but that would spoil all the fun...
This page contains spoilers! Read on your own peril.
The elemental wards were a set of four Magical locks who each represented one of the four classical elements, namely air, earth, water and fire and each one contained a corresponding talisman that combined acted as a key to unlock the wards.[1] They were kept by the Keepers to contain destructive forces in case some kind of disastrous outbreak would occur.[2]
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The wards on the cathedral door.
The wards were placed on the doors of the abandoned Hammerite cathedral in the Sealed Section of Old Quarter by the Keepers after the incident to seal whatever evil lies within and hid the talismans in various parts in The City (and even outside in Thief Gold) to prevent another such catastrophe to occur.[2][1] At the time, the elemental wards were a rare instance of the Keepers taking an active and overt role to change the course of events.[3]