Cerulium was a mineral found on Ceres which had the property of inhibiting free will. The native Cerulians used it as the basis of their civilisation until they were destroyed by an asteroid. However, chunks of cerulium remained in space, some of which fell to Earth and were recovered by the Department.
Inspector Drake planned to use the cerulium to control the people of London. He introduced cerulium bracelets to the students of Magdalene Academy and developed an experimental cerulium-powered weapon called the Inhibitor. However, the Inhibitor exploded when it was tested, and the bracelets were destroyed by K9. (TV: Sirens of Ceres)