This is a quest given by Archmaester Castos at the Citadel's archives in Oldtown.
He tells you to talk with Malko who gives you a quest about finding a septry north west of the Vale
The septry can be found east of the village of Fogfield
after talking with the local septon he tells you to talk with Ser Elston
when you have atleast 2 men travel to the Barrow and defeat a crazy cultist.
after this go back to Malko in King's landing
500 coins, 1500 experience, 5 renown