Grayton ghost

In the city of Grayton's Lot on the Karenth Centralis, Continent of exist a number of hushed rumors and myths about a ghost that haunts the city of Grayton's Lot and the city castle. It is said that this being was one of the early Grayton family members, or a bastard son of one of the early family members. The being comes out at nights when there is a full moon or wanders the halls of the castle in the cold winter nights.
The truth is that the ghost is that of man who took one of the early Grayton daughters out of wedlock and she bore a son to him. The Grayton's Lot mayor found out that this happened to his daughter and tortured the man to death in a secret room under the city. The man's spirit cannot rest until he is avenged, but is torturer is long since gone.
It might be possible to apease the spirit by showing it proof that its tormentor is gone.
The ghost is only seen around the city a few times per year, as the spirit is weak and cannot manifest often. When it does, it brings about all the rumors and hushed warnings throughout the city for the next several weeks.
The ghost is a weakened form of ghost only aging a handful of years and will not fight for long before fleeing.
This ghost would be a great plot hook or sub story that could be intedgrated into adventures being held in Grayton's Lot, especially intrigue role play with the castle and government entities. Solving this issue could bring great raporte with the members of government, clergy, and the community as a whole.