Ereticy

The Ereticy was a Russian witch-cult. According to rumour, it was patronised by Empress Catherine the Great during the 18th century. It was one of the thirteen groups invited to the Eighth Doctor and Scarlette's wedding in 1782; while they did not publicly attend, Katya was believed by the others in the Henrietta Street House to be a Russian spy. (PROSE: The Adventuress of Henrietta Street)

Behind the scenes

 * The word "ereticy" is a variant on "eretik", a type of vampire in Russian folklore.