An actual, historical artifact, that holds a terrifying,
unsuspected secret.

The material between the brackets is true, taken from http://www.hiddenlondon.com/st_sepulchre.htm

Seen, but not touched! If this dire artifact is rung in the prescribed manner, and the rhyme is recited at midnight, all who hear its dreaded toll (with their own ears – broadcasting, recording, etc. will not work) will die the next day before noon, including the crier. The deaths will appear to be accidental but will all involve either drowning, a broken neck, or strangulation/choking. The only way to avoid this fate is to have the condemned formally pardoned by the rightful occupant of the English throne (currently Duke Francis of Bavaria, certainly NOT that Hanvoverian usurper on the throne today!)

If the Sleepers become aware of this one…right under their noses….