BUT ghosts are not monsters. That goes for spectres, spirits, haunts, apparitions, and all the other Synonym Monster Gary and Dave and the Hair Bear Bunch threw into adventures and expansions over the years, leading to that one thing oh so worse than pointless monster bloat, pointless ghost bloat, the new album from Kraftwerk. I don't mind poltergeists since I think of those as more a nature spirit/demonic presence, and you can give me all the different iterations of undead all day long. Sure. Give me every culture's bump in the night. But ghosts are above all that, frequently also partially interposed with all that.
Wraiths are a very specific thing. D&D wraiths at least are drawn (like many D&D monsters) from one, very, specific, source, but not so specific that it's not also the reason we have Orcs. They also represent an important presence in the world if only by implication.
When a powerful cleric uses his divine protection to cheat the very god he serves and extend his life past the point of death, that makes a Mummy. When a Magic-User does the same, that makes a Lich. When your soul returns to your body, or uses a nearby body, and animates it with your faculties and memories to seek fulfill some task, usually the seeking of vengeance, that's a revenant. When the mind persists in a body without the soul and the flesh whithers, that's a wight. When a soul is so corrupted that it warps the mind and destroys the body, it makes a vampire. When a soul bound for the spiritual plane is so distraught that it returns to physical form through the magical plane just to be able to voice its sorrow, that's a banshee. Down and down the turtles go.
Like all Gygaxian Synonymism every user has to find their own breaking point. I'm pretty on record about ghosts already but ghouls, to use another example, I prefer to either be something any normal, living creature can devolve into through practice of feeding or magical affliction, or else just make them a separate class of creature altogether. Similarly no one should ever fight a naked Wendigo, you should always be fighting someone with a Wendigo inside of him, and it should be treated almost like a werewolf curse except you incur it by knowing of the curse or by committing cannibalism in an area consecrated against the Wendigo.
Wraiths are mortal (not Elf) souls sustained beyond death by someone else's magic. That's not my definition, that's basically Tolkein's definition, and it jives fine with the description in BX. Someone casts a spell and binds you here to their service, or you have some magic item ooky ook whose hold must be broken to dispel this menace, etc. The soul always resists, which makes it go all Taken on people, which means that even if those held in sway aren't corrupted or evil already then the difference on the ground is negligible to your Thief level 4.
A wraith's closest ancestor, then, is a skeleton, and here I get to both quote myself and expand on an arm of my unified field theory of monsters, so enjoy my enormous balls.
From my dog book:
They are driven not by any intelligence but, like all animate skeletons, by a will. They run without their legs properly working, hunt without sight or smell, and crush flesh and bone between yellowed teeth without the aid of powerful neck muscles. They are an extension of their creator's guile given absence-of-flesh and set to work like a hateful machine.
And they are hateful, for they are infused with an inkling of their former sentience, just enough to feel a constant low-level fear that things are not as they should be. They hunt their peace forever. They rot until they're dust. Then they coat the floors in mild terror and malevolence.
I've talked before about how a lot of different kinds of monster are basically the same monster for me. Well another part of that is the idea that a ton of monsters in the manual are spells. I don't mean A Wizard Did It in the traditional owlbear sense, where somebody wound that particular watch and it has been ticking away every since. I mean it in the sense that if you encounter these things it is because one specific person very much intended that you do so, or that someone do so. These spells may have been cast out of sheer paranoia or you may be ultimately caught in a crossfire, enduring a spell effect cast two hundred years ago to deal with an enemy from one hundred years ago.
I prefer my zombies to be voodoo zombies, generally. That's a spell effect. But even I have room in my heart for a high level caster raising a whole cemetery to aid them, where we're talking spell effect, and it's mostly because of how much cooler SKELETON armies are, and how, on a long enough timeline, both Bela Lugosi zombies and George Romero zombies will end up as Harryhausen skeletons.
Somebody set the machine a-workin'. They gave it more fuel than it would ever need, enough fuel to drive the machine far past the point that all its components busted and rusted away. It will run basically forever and, without even the capacity to EXPERIENCE horror, will radiate a sense of doom at the particle level. You will breathe in their doom.
Skeletons do that but everyone has a skeleton so they're familiar enough to inspire mortal fear but not dread, for these purposes the distinction being anticipation vs helpless anticipation. They also, as mentioned above, break. Wraiths don't have anything to break. They kept going as one by one their breakables fell off. Because of that there's an inescapability. Flying and passing through walls and shit only helps this, although I don't actually think wraiths should fly. But see skeletons make more dead guys but not more animate skeletons (except in my games, where skeletons killing you basically punches your skeleton out of your body and then it wanders away to join the skeleton kingdom). Wraiths do.
Wraiths are a spell effect that wraiths learn by becoming wraiths. I'm in an endless unliving hell? Oh I get it, huh I'm super pissed off and this looks like fun. Wraiths beget wraiths, like stabbing Frodo with a morghul blade woulda, and they do it because why waste the magic to bind an army of spirits when you can bind ONE spirit and then let HIM give up his Sundays building your army? Now you can evil golf, work smarter not harder.
That's the best thing about turning monsters into spell effects. You're not just dealing with that creature's kinetic energy but also its potential. With some monsters their threat looms until they show up and then they must only be endured. Wraiths give you both. Everything people like about undead urgh brains zombies is better with wraiths. All spell monsters are improved that way. Shadows, cast it on an object, it attaches to the first person who uses the object, animates their shadow to kill them, then that shadow infects another shadow, it just keeps going. Doppelganger, cast it on a person, they turn into a perfect copy of a person you intend, and are consumed by a bottomless drive to find and kill that person and assume their life, after which they may themselves cast doppelganger as much as they please.
All of this gives you that broader context of "Someone specific in the world is trying to kill someone else specific," someone who may or may not have anything to do with you. It tracks these creatures not as naturalistic facts of life in a magical world but as dedicated boobytraps. RPGs are the one place where rocks fall everyone dies can be not just a good ending but the best ending because, usually, rocks fall everyone dies was set up INTENTIONALLY by someone in the crypt or dungeon, or it's a direct consequence of your choices and actions in electing to explore the dwarf tunnels. A bunch of skeletons is cool, if anybody ever says otherwise I fucking hate them, why did someone say that, who said it, I will punch disease in their organs, but someone's skeleton army is always going to be cooler. It gives you about four other directions to go for more story than the bunch of skeletons smoking in the parking lot does.
This is why ghosts are still not monsters but wraiths are. Because wraiths aren't just the ghosts of somebody, they're somebody's ghosts, closer to a bound demon than Uninvited. Wraiths don't give any fuck about invitations.
Imagine a world then where a powerful necromancer blanketed the world with his wraiths. He would be subtle, mostly, but with great tectonic cataclysms of them, sending them there to waste enemies and melt people and every act grew his power. He would never let you see them, these wraiths, unless it was known you might not be believed, or might not believe yourself. He could sway whole nations like that without the wraiths ever striking. He could help cultivate his legend and command the respect of millions, billions even over the centuries. He could create a whole second world, a life after life, all under his sway, all in homage to himself, and he could even make people believe THIS IS ALL A GOOD THING. If there were such a thing as ghosts, sure, but there weren't ghosts, if every one was a wraith. He'd be above even the mightiest kings and his influence would spread further and run deeper, subtler, stronger throughout the millennium, why he'd be worshiped as a...