Danu wearing her crystal transmitter.

A Crystal transmitter was a hidden transmission device employed by Princess Danu during the Drogheda Civil War. She used it to communicate with the Drogheda Revolutionaries undetected, as she was spying within the Drogheda Citadel, and needed to remain inconspicuous. The transmitter was worn around the woman's neck, where she was able to pass it off as a necklace.