In my Ghostbusters-inspired RPG the player characters found out that the main haunting wasn't actually the ghost of a janitor, but the much more powerful thing that had affected him with its rage. The entity manifested itself in the shape of a huge spectral wolf, that attacked everything within sight. The creature was revealed to be the manifestation of a tribe of Native Americans, who had died on a field of battle in the 17th century against early settlers in modern-day Boston, with the tribe's shaman becoming the dominant personality in the cluster. The restless spirit had become frighteningly powerful due to its grief over losing its home, but it usually laid dormant unless disturbed by lightning striking the ground near its earthly remains. But a hotel had been built on top of the former battle-field and the new electric-system bothered the spirit so much that it started to haunt the hotel, as well as turning other usually peaceful ghosts into angry monsters like itself.