Raging Ghosts are a strange form of restless spirit projected from the Twilight World of the dead into the Mortal World of the living. To Scarlett's normal vision, these can be vaguely seen as a large, translucent, flickering violet light, resembling a giant candle flame. Despite that appearance, they can still reach into the mortal world and inflict harm on Scarlet there. She, however, cannot affect them from the mortal realm.
When Scarlett uses her necromantic skills to invoke The Passage, her view from the Twilight World is quite different. Here, these creatures appear as if some sort of giant prawn, with a dozen flailing feelers or tentacles. At least from the Twilight World, Scarlett can fight back. Despite these creatures connection to the dead, any weapon Scarlett wields against them in the twilight world can do them damage, and they are also vulnerable to her necromantic Mental Skills.
Scarlett can only see and attack Raging Ghosts from the Twilight World when she uses The Passage. Whether Scarlett is using her the chromatic site with The Passage or not, a Raging Ghost's many flexible feelers provide multiple attacks with a long reach, and they can do considerable damage. So, it's imperative that as soon as Scarlett gets close to a Raging Ghost, she invoke The Passage to defend herself (or, she can flee.) If Scarlett's time in the Twilight World expires before she has defeated all Raging Ghosts nearby, she will immediately become defenseless; Enduring Passage, which extends the duration of The Passage, can be useful in facing multiple Raging Ghost enemies.
Defeating a Raging Ghost adds 170 points to Scarlett's Experience. Nothing of value can be found by searching the remains of a defeated Raging Ghost.
Scarlett will probably encounter Raging Ghosts for the first time while exploring the Catacombs of the Arsenal District. Later, they may begin to appear on the streets of the Arsenal District, and can even be found all the way to the Outer City. (Very late in the game, they seem to spawn outside the doors of what once were Rogues lairs. See e.g, Chasing Bandits in the Outer City, Chasing Bandits in the Outer City, or The Lesser Evil quests.)