It starts when you find a dead body on the road near the cemetary. If you follow the trail of blood it will lead you to one of the gravestones. Read the gravestone and a Ghoul will eventually attack you.
Defeat the Ghoul. Nightshade murder is the only sidequest where you don't receive a reward, but the murderous spirit will be gone!
After doing this, if you walk out from Nightshade Cemetary and then come back you will see Gideon standing in the cemetary.