A device made by Mer'Eric the Walker.
Built on the highest point of the Brestian Hills, it is basically a giant magnifying glass, using a complex mechanism commanding several mirrors, that is able to concentrate the sunlight into a powerful beam.
The concentrator is associated to a Sun Stone that can receive similar long-range light signals and transform them into pure thaumic energy.
A keyboard is attached to a mechanical obturator, allowing a single operator to send coded messages to very long distances.
A magically-powered scriptor writes any such incoming message capted by the Sun Stone.

Although Mer'Eric only saw the Great Heliograph as a research project and as a way to communicate easily across huge distances, this device is also a powerful weapon that can burn nearly anything several miles away.
This is probably why he decided to hide it in a sealed cave.

A similar device was built by the Mad Mage of Scorn , but its fate is unknown.