An Oprichnik (опричник) (plural Oprichniki) was a member of an organization established by Tsar Ivan the Terrible to govern the division of Russia known as the Oprichnina (1565-1572.) It is thought by some scholars that it was Ivan's second wife Maria Temrjukovna who first gave the Tsar the idea of forming the organisation.
The Oprichniki were responsible for the torture and murder of internal enemies of the Tsar. Notorious for their violent means of enforcement, they could be compared to modern "death squads" or even secret police. Guided by Ivan, they laid waste to civilian populations. They dressed in black garb, similar to a monastic habit, and bore the strange insignia of a severed dog's head (to sniff out treason and the enemies of the Tsar) and a broom (to sweep them away). The dog's head was also symbolic of "nipping at the heels of the Tsar's enemies." They were sometimes called the "Tsar's Dogs" on account of their loyalty to him. They also rode black horses in order to inspire greater terror. The Oprichniki were given orders to execute anyone who was disloyal to Ivan IV. Their most odious member was Malyuta Skuratov.
The Oprichniki would use different methods of torture including tying each limb to a different horse and riding in the opposite directions or dropping the person into a vat of boiling water. They would impale victims, or even tie the victim to a pole and burn him over an open fire. All of this was supported by Ivan IV (Ivan the Terrible).
When Ivan declared himself the "Hand of God", 300 of the Oprichniks were selected to be his personal "brotherhood" that lived within Ivan's castle. Every night at 3 am these Oprichnik "monks" would attend a sermon given by Ivan himself before the morning's ritual executions. The Oprichniki would lead an externally ascetic lifestyle, like the monks they emulated, but there would be rash outbreaks of cruelty and debauchery.
In the infamous Novgorod incident, the Oprichniks massacred an estimated 1500 presumably innocent "big people" (nobles), although the real figure is unknown.
The song "Dog and Broom" on Arghoslent's Hornets of the Pogrom album focuses specifically on the Oprichniki.