The Nightshade quest concerns the ghost referred to as Agdan. It can be acquired in several ways - either by encountering the ghost in the cemetery, by finding the secret cellar in Soko's Barn, or by talking to Ella regarding the Golem and the guardian's rest.
This side-quest can be completed by a player on any of the three major paths, since helping Agdan, the bladespectre, or the Golem to find peace furthers the goals of both the good and evil NPCs in the game.
Agdan cannot rest because not all of her personal belongings are buried with her. In order to find rest, Reylyr and Agdan must make peace with one another. Alternately, Agdan must be banished from the world. The bladespectre alone is no longer strong enough to defeat Agdan. If he is to cast Agdan out and restore peace to the land, he will need the Golem's help.
In order to be at peace with one another, Agdan must know that Reylyr now regrets her betrayal. Somehow, some of Agdan's things must be buried in the graveyard. But all of her things are hidden in Soko's Barn. Only the key to the chest can be found in the cemetery, and Agdan has hidden it.