Haunts (2016)

From the introduction:

Haunts are a [Dungeon Master's] way to spook the players and create mood for a haunted locale. Haunts are not monsters like ghosts or specters, they are more like an echo of a lingering tortured or very willful individual or group. They’re not traps either, they can’t really be detected until someone triggers it, because then it is dormant and not even really there. It is suggested if a character uses an ability to detect the presence of undead, haunts would count as such. Whenever one is triggered, it always has an associated visual or audible effect, or both. If there’s a visual it can be translucent, possible giving away its nature or as realistic as needed for the scene to play out.