The Armilla was a bracelet made of mercury that was worn by Wainwright.
After Wainwright was killed by Margie del Monsalvat, the Master took the armilla. He hoped to use it to control the Godhead.
Instead, the mercury from the armilla animated the dead body of Father Sherrin. Under his control, the Godhead left Earth. (PROSE: The Duke of Dominoes)