Wraiths have a strong visual identity: hooded, faceless black figures. They are the Grim Reaper, the Ghost of Christmas Yet to Come, the Nazgul, even the Dementors (if you must). They look scary. Everyone knows the empty cowl means death. 

My own drawing

D&D doesn't know what to do with them. Are they corporeal? Do they paralyze or Energy Drain? Are they Nazgul or is that Spectres? It all depends on which edition you prefer. Delta has a great round-up. 

With their intimidating visages, wraiths should advance the thesis that I've explicated in previous posts (skeletons, zombies, ghouls, wights) that each undead type ought to be uniquely terrifying. 

My wraiths are of the semi-corporeal, black mist mode of wraithdom. They'll look solid enough, but your glaive-guisarme passes clean through. Unlike wights, wraiths don't have names or personalities. They are anti-life voids, avatars of nihilistic hatred. They just want to snuff all the candles. 

The Lord of the Rings: The Fellowship of the Ring (2001)

Death Spirals Revisited

These guys are brutal. They've got auto-hit life drain (don't worry, I won't make you deduct levels), dissolution at 0hp (no save, no res) and escalating damage that snowballs like a motherfucker.

This monster sets up tactical puzzle-fights and is a bit more mechanically complex than my previous undead. They can float over obstacles and flow between people, so fight positioning will be unusual.

Instead of the canonical level-drain, I've given the wraith an area-of-effect aura. Wraiths deal damage to all adjacent creatures, with no roll and the monster gains the hit points taken. This damage increases every round. The party needs to win fast or it will get out of hand. 

The goal here is for the players to fear them. For their arrival to be an "Oh, shit!" moment, without having to resort to level drain.

Muppets

Wraiths

Armor Class: 15 (It's hard to know where in that flapping cloak to stab)

Damage Immunities: No damage from mundane weapons. Half damage from magical weapons and most spells. Normal damage from fire, blessed weapons and holy magic. 

Hit Dice: 4d8 (HP 18)

Attacks: Draining Aura 

Move: Float. Speed as an encumbered human. Moves at the end of the round instead of initiative order.

Semi-Corporeal: Can flow through an arrow slit but can't pass through walls or a close fitting door. 

Saves: 12+

Morale: 9. Check morale if damaged by fire

Number Appearing: d4

Draining Aura: Creatures within 5' of any wraiths take damage with no save. Damage starts at a d4 and each round thereafter is d4 + the prior round's damage. The wraith regains the hit points taken.

• Roll damage once per round and characters next to any number of wraiths take it. 
• If more than one wraith is present, they divide the hit points gained.
• The aura counts as cold damage for the purposes of resistances etc.

Deprivation (Warmth): Creatures affected by the Draining Aura cannot benefit from mundane healing until they have warmed by a fire for a few hours. 

Dissolution: A creature reduced to zero hit points by a wraith dissolves into ash and flows out of its armor as a newborn 1HD wraith. 

Devour Light: Mundane lights decrement in rounds rather than turns in the presence of a wraith.

My post-it doodle

Suggestions for Running the Wraith

This monster has a lot of pieces to track, so a few pointers to run it smoothly:

The wraith's split actions, attacking in initiative order but moving at the end of the round, is unusual. The aura hitting everyone affected simultaneously is also weird. This gives a wraith fight a different cadence than the usual exchange of blows and spells in initiative order.

The action is driven by the wraiths moving at the end of each round, then a tense initiative roll at the top of the round. Characters that beat the wraith act first and can get out of the aura before it triggers. Players that don't win initiative take aura damage (healing the wraiths) and then act. Finally, the wraiths move again.

If you don't roll initiative each round in your system you'll want to adjust how this flows.

Remind players to decrement their light sources every round. 

Even if you're usually a textualist, this is a good time to let spell descriptions slant rhyme. Desperate solutioneering is the desired behavior here. Don't quash it.

A few illustrative, generous bends (comparing to the OSE versions): 

• Light - cast in its blinding form, will force them to flee on a failed save (despite no eyes)
• Bless - deals d8 damage if cast on a wraith, otherwise normal effects
• Protection from Evil - shields vs. the wraith's aura. Count it as a melee attack from an enchanted creature. If Protection from Evil doesn't work here, what is it even for?

Tactics

Wraith tactics are simple. They flow between people easily and position to maximize the number of targets within their aura. 

For players this is likely jarring. Their shield wall is meaningless. There are no "frontliners" in a wraith fight. The obvious move is to scatter, but this comes with its own downsides, e.g. if they need to heal or flee.

While they're not fast, wraiths are unimpeded by terrain obstacles. Cover is effectively impossible. Bottlenecks will be dangerous.

Escalating, multi-target autohits will get bad fast. If the first couple rounds don't go great the players should be ready to run. 

If the Turn lands, the Fireball rolls high or the Blessed Blade of St. Albion crits, and the fight resolves easily then "Remind yourself that overconfidence is a slow and insidious killer."

Fear-mongering aside, if players come in stocked with Holy Water, fire spells and blessed weapons, wraiths will melt fast. They just take a different toolkit than a roomful of orcs.

LotR: Fellowship (2001), again.

Common Knowledge

Wraiths are very rare (Thank Pelor!) and those who have seen them are now mostly wraiths themselves, so knowledge is scarce. Still, those who have studied the scriptures or listened to the oldest folk wisdom might recall helpful fragments:

"Howbeit this kind is not balked but by prayer and fleeting."

"When the dark shrouds drift,

    Repent and shrift"

"When darkness looms and shadows attack, 

    Sometimes fire can beat them back."

"Torches dance as they approach, and roosters crow at their passing."

"A burning bulwark is the sun,

A beacon never fading.

Like our bright star, we shall not run,

No shrinking or evading.

Take up thy saintly blade,

Let not thy candle fade,

Let blessed waters flow,

And holy brilliance glow,

Lest starving dark consume thee."

"Q. Wherein consists the nature of the estate whereinto they fell?

A. The nature of the estate whereinto they fell consists in their formlessness, their vexation by the living, and the corruption of their presence.

Q. What offices doth a cleric execute to humiliate these spirits?

A. To humiliate these spirits, the cleric executeth the office of lighting their eyes, the office of protecting from evil, and the office of turning the dead."

These bits have broad overlap with other undead. I had fun writing them and they should be easy to re-purpose.

My own drawing

Biology

Wraiths are the corrupted spiritual residue from deaths that left no body, just a spirit-shaped void. The most common origin is people consumed by magical fire or disintegration but sometimes they're spawned by teleportation failures, failed summonings or other magical accidents. Rarely, by spiritual mishaps (brimstone, etc). 

At the site of strong magic or divine intervention, the membrane of reality is stressed, and the sudden pressure of carnal-cosmic bonds snapping can create a pinhole leak. Through that tiny tear, the insatiable malice of the Outer Dark leaps to fill the broken vessel, but finds no physical purchase. 

They are not demons, they are too simple to be chaotic, but they come from the same source.

If you really want to make your players think twice about slinging fireballs and lightning bolts at every party of bandits that crosses their path, give them a chance of creating wraiths. One in a hundred for every person vaporized. Roll whenever you remember. 

When wraiths dissolve a person, they are reproducing the conditions of their own creation. A new wraith will immediately seek to feed. That's what newborns do. 

After consuming a few adventuring parties, a wraith will not be a 150 hit point black colossus. They can only get so big. The most they can absorb is the maximum of their hit dice (4d8 = 32 hit points). This is plenty bad.

Wraiths are not trying to grow strong or to propagate. They don't want anything. They just drain anyone that gets near and then drift around doing nothing until someone else gets close. They are extra-planar evil pollution.

Epiphenomena

Silhouettes burned into the wall are a bad sign.

When wraiths are near, torches flicker wildly and cast spasming, jagged shadows. Lantern flames beat at the glass like a trapped moth. Candles burn down rapidly, spattering wax. 

Sounds flatten. 

The musty dungeon smells disappear. The air isn't clean, just empty. 

Dementor, source unknown

Encounters

Because the wraith's abilities are so spatial, the shape of the encounter location will particularly matter. You can get a lot of mileage by varying fight geography. A big open room vs a warren of small ones will lead to very different dynamics. Put them in tunnels, on bridges, in towers full of ladders. Put them in swamps, on cliffside switchbacks, and near perilous climbs. This is true for all encounters but especially for wraiths.

Parties can't outrun a wraith if they are encumbered. It'd be a real shame for one to show up while they're hauling all those chests of gold from the grimlock's lair.

They're a pretty good jump scare. Drifting up from a chasm while backs are turned. 

A few other options:

• A room filled with amphorae, each containing a wraith. A trap! A resource! A mystery! A really bad place to start a fight. 
• The party was hired to retrieve the Murgahl-knife from a dungeon, only to discover that anyone killed with it dissolves and becomes a wraith. Do they give it to their patron?
• This labyrinth has more wraiths than they have spell slots and holy water. Can they get through?

Loot: 

If you could bottle a wraith, someone will want it.

When a wraith dies it leaves behind a soot impression, like a charcoal drawing of a screaming face. If swept up, it's a deadly poison.

Wraith Soot: Poison. Looks like laser printer toner. If inhaled, 4d4 damage. If ingested, Save vs. Death. On a success, the victim is reduced to 1hp. On a failure they dissolve and become a wraith.

Occasionally, a wraith leaves behind a single blackened bone. These are prized as focus objects for necromantic magic.

Bones of Wraith: When used as a focus for Necromantic spells that deal direct damage, add an additional die

If you feel compelled to drop more loot:

• A magic ring is very thematic
• An ancient, poisoned dagger also has precedent
• An indestructible black gem. Looks like a jet egg. If swallowed, you can levitate (as the spell) for d6 minutes. It'll be a couple days and some unsanitary searching before you get it back.

AD&D Monster Manual. Sutherland

Variants & Re-skins:

• If you don't like the cowls, the Sutherland look is also pretty cool. 
• The Wraith-eon: Casts magic missile as a 5th level magic-user every round. A byproduct of the military-magical complex. The archmage Eisenglower warned us about this. 
• Bound Wraith - If a wizard can get control, they're a fearsome servant. Like Hethor's notules.

Discussion

I started my wraith draft right after finishing my wights post, so I had Middle Earth at the forefront of my mind. I set out imagining that I'd be statting up an incorporeal ring-wraith but then I had the idea that the notules in Book of the New Sun were kind of a wraith and from there, things drifted in a different direction. 

The version of the wraith I landed on should still fulfill the usual niche for this monster, as a scary mid-level threat, but with a weirdness not present in the original.

Wraiths should be more than just faceless blurs between Shadows and Spectres on the incorporeal-undead escalation curve. Fighting them should feel threatening in a specific way: dangerous, sure, but also strange. Their cosmic backstory should add an uncanny edge.

A Lovecraft void in Tolkien's cloak.