Ghosts, in my opinion, are incredibly straightforward with regards to how they are supposed to be used: They always, always, come with a mystery attached. How that mystery plays out, and how the ghost itself manifests, leave a lot of room for creativity, but the basic setup remains the same.
Further pushing this highly individual approach to ghosts is the fact that they aren't a single stat block - though we are given the commoner and mage variety, ghosts come with a monster making template attached, giving GMs a wide variety of options for making a ghost's stat block a perfect thematic fit for the mystery they come attached to. They are also one of the few creatures that come with alternate conditions to "beat" the encounter baked in; Rejuvenation notes that a ghost only is permanently destroyed when it has been lain to rest. Bear this in mind when utilizing the ghost.
Hook 1 (TJ) - The Stalker of The Dunnhills
Following a particularly catastrophic fire, the trees of the Dunnhills Forest are nothing more than stumps and soot. As for the wildlife, those that could not escape the blaze perished as well, their lingering fear and anguish adding to the oppressive atmosphere of residual heat and devastation. If your path takes you through the scorched remnants of the Dunnhills, you might notice a flicker of movement here and there; ghostly rodents are still going about their previous lives, finding places to hide that exist only in their memories, seeking shelter in canopies that no longer stand. Over several trips, you might notice that the number of these apparitions is dwindling. With the lack of movement underfoot, the forest of yesterday is quiet as a graveyard. All this seems quite natural.
But this is no longer a natural place.
You may no longer see any movement, but the ghostly revenant of a mountain lion, stalking paths only it knew, is running low on mice and rabbits to eat.
So, while you do not see it; it certainly sees you.
Hook 2 (Max) - Unfinished Business
The Jermainn merchant family just can't seem to catch a break. Not long ago their patriarch, a man of considerable skill and wisdom, succumbed to a mysterious illness, only to be followed by an unfortunate accident that struck his only male heir who suffered a substantial blow to his cranium.
Since then the young man changed, considerably so. He has been acting more erratic and, at least according to his mother, far too much like his late father.
Hook 3 (Nemanja) - Finnick's Finale
Finnick the Fickle was a gnome sorcerer who was long ago imprisoned by the mad dero. For a full year he was experimented on with little regard for his life or well-being. Finally, he gave in, and tried to end the agony he was in, using the last reserve of magic left in his body to burn the entire laboratory-prison he was in to the ground, taking his own life in the process.
Alas, it was not to be. Three days later, he rose as a ghost. While the mage has long since slain all of his captives, three copies of the journal accounting the Grand Alchemist's experimentations were passed down to his grandchildren, each of whom now serve as Grand Alchemists of their own conclaves. The spirit will not go to rest until those books are burned down, though a more morally dubious finder might realize those could be pretty valuable to scholars of anatomy or even necromancy...
