Hide and Seek

Walkthrough
If the player has sided with the Dawnguard, Gunmar sends the Dragonborn to kill a vampire masquerading as a Travelling Pilgrim at a radiant location. Because the guards see him as a pilgrim, killing him will be treated as a criminal act.

However, if the player has chosen to fight alongside Lord Harkon, then he is sent to kill a Dawnguard soldier pretending to be a Traveling Pilgirm at a radiant location.