A Jarl's Justice

Walkthrough
Isran at Fort Dawnguard has become aware of subversive vampire activity close to the rulers of the holds, i.e. the Jarls. The latter need to be warned of this threat and the "Advisers" removed from the picture.

Warn  about the vampire threat
Travel to the Jarl's long house in the indicated town and inform them of the vampire threat. The Jarl will listen to the Dragonborn, but will not act unless there is definite proof.

Find evidence linked to the vampire
The target is usually right there in the long house with the Jarl, sneak up and pickpocket the vampire. This yields Orders to the "Visiting Advisor".

Alternately:: Discreetly kill the target. Sneaking and using a bow, may require the right time of night to pull off.

Bring evidence to 
Bring the Orders note to the Jarl, he will allow the execution of the target and the guards will not interfere.

Destroy the vampire masquerading as a Visiting Advisor
Kill the vampire.

Examples:
 * Warn Skald the Elder in Dawnstar at The White Hall, reward Glass Bow of Blizzards or Necklace of Extreme Sneaking.

The only other location is Riften.

Return to Isran
Return to Isran and receive a reward.

Since these are Radiant (randomly generated) quests, this quest can be repeated endlessly.

Ein Jarl spricht Recht