Oprichnik

Oprichnik (, , "man aside"; plural "Oprichniki") was the designation given to a member of the Oprichnina, a bodyguard corps
established by Tsar Ivan the Terrible to govern a division of Russia from 1565 to 1572. 
Foundation.
Some scholars believe that Ivan's second wife, the Circassian Maria Temryukovna, first had the idea of forming the organization. This theory comes from Heinrich von Staden, a German oprichnik. Maria Temryukovna's brother also became a leading oprichnik.
Oath.
Upon acceptance, the new Oprichniki were required to swear an oath of allegiance:
I swear to be true to the Lord, Grand Prince, and his realm, to the young Grand Princes, and to the Grand Princess, and not to maintain silence about any evil that I may know or have heard or may hear which is being contemplated against the Tsar, his realms, the young princes or the Tsaritsa. I swear also not to eat or drink with the zemshchina, and not to have anything in common with them. On this I kiss the cross.
Description.
Modern theories suggest that the motivating purpose for the organization and existence of the Oprichniki was to oppress people or groups opposed to the Tsar. Known to ride black horses and led by Ivan himself, the group was known to terrorize civilian populations. Sometimes called the "cromeshnina" (selected) because they were a hand-picked body, the Oprichniki dressed in black garb, similar to a monastic habit, and carried attached to their saddles a severed dog's head (to sniff out treason and enemies of the Tsar), or an actual wolf's head and a broom (to sweep them away). The wolf's head was also symbolic of the hounds of hell tearing at the heels of the Tsar's enemies. The logistics of acquiring the canine heads was quite gruesome. Due to the lack of taxidermy, the severed and drained heads would only remain frozen for the winter months of the year. To maintain their image, the Oprichnik required a constant supply of fresh heads. Ivan himself carried a fearsome canine head made of iron with jaws that would open and snap shut as his horse galloped.
The Oprichniki were ordered to execute anyone disloyal to Ivan and used various methods of torture to do so, including tying each limb to a different horse and riding in opposite directions, death by boiling, impalement, and roasting victims tied to poles over an open fire.
When Ivan declared himself the "Hand of God", he selected 300 of the Oprichniki to be his personal "brotherhood" and live in his castle at Aleksandrovskaia Sloboda near Vladimir. At 4 a.m., these select Oprichniki attended a sermon given by Ivan, then performed the day's ritual executions. The Oprichniki sought to lead an externally ascetic lifestyle, like the monks they emulated, but it was punctuated by acts of cruelty and debauchery. Ivan sang while his Oprichniki ate, eating only when they had finished. At 9 p.m. he went to bed, listening to stories told by three blind men.
In the Novgorod incident, the Oprichniks killed an estimated 1500 "big people" (nobles), although the actual number of victims is unknown. By 1572, the Oprichnik had become a destabilizing force and were disbanded by Ivan, especially as he blamed them for the lack of response to the Tartar sack of Moscow in 1571. Thereafter it became a capital offense to say or discuss "Oprichnina".