The Charn Bell was a magical bell, which hung from a little golden arch, fixed on top of a small, square pillar that was about four feet high. There was also a small, golden hammer, which lay beside it, for people to strike the bell with.
On the pillar was an inscription that read:
- Make your choice, adventurous Stranger;
- Strike the bell and bide the danger,
- Or wonder, till it drives you mad,
- What would have followed if you had.
The pillar was situated in the Hall of Images, in the city of Charn. It was placed there by Jadis shortly after she cast the Deplorable Word, which destroyed her entire world, in order for her to defeat her sister. With no one left to rule over, and no life on Charn, Jadis used her magic to place herself into a deep sleep. The bell was apparently part of this spell.
There's no mention of how the bell first came into being, so Jadis could have created it with her magic, or she could have simply used any old bell that she cast a spell on.
The purpose of the bell was to awake her from her sleep, as soon as someone rang it. This happened when Polly Plummer and Digory Kirke were accidentally transported to Charn through the use of Magic rings.
When Digory read the inscription on the pillar, he was unable to resist the temptation, and chose to ring the bell, which caused a continuous sweet note that grew louder with every passing second. Before long, its sound had grown to the point where the stone floor trembled beneath the children's feet, upon which, Jadis awoke.