Deceiving the Herd is a side (Radiant) quest in Skyrim: Dawnguard. This is a repeatable quest.
Deceiving the Herd is only available to Vampire Lords.
Deceiving the Herd starts in Castle Volkihar
Deceiving the Herd is given by Feran Sadri.
Feran Sadri wishes to sow confusion and anger amongst the people of (Random Hold Capital). I am to kill a citizen out in the streets while impersonating a member of the Dawnguard. Then I am to place an incriminating letter on the corpse. The higher the profile of my victim the better.
Kill someone out in the streets of (Random Hold Capital)
(Optional) Choose a High Profile Victim
(Optional) Make it a public kill while wearing Dawnguard Armor
Plant the Incriminating Letter on the victim.
Wear the Dawnguard Armor
Go to the city you were assigned.
Find a High Profile Victim who could be someone attached to the Jarl's court, a merchant, a bard, a priest, or someone with stature
See here for a list of High Profile Targets:
Kill victim in public, in plain sight
Plant Incriminating Letter on victim's body
Return to Feran Sadri in Castle Volkihar.
Potion of Blood (3)