Wraith

Wraiths are shadow-like, incorporeal entities who in their former life were debased or entered into a fiendish pact, consigning its soul to eternal damnation in the evil centered Outer Planes.

Description
Wraiths have little few memories left of their mortal life and even the strongest events/emotions become fleeting as half-remembered dreams. In rare moments, they "might pause to stare at something that fascinated it in life, or it might curb its wrath in acknowledgment of a past friendship," most of them despise any reminder of their past and the unfortunate transformation.

Characteristics
The creature is suffused with negative energy: its mere passage leaves nearby plants blackened and withered, animals flee from its presence, and even small fires can be extinguished by the wraith.

Wraith are often resistant to acid, cold, fire, lightning, thunder damage and nonmagical damage from bludgeoning, piercing, and slashing attacks that aren't silvered. They are also immune to necrotic, poison damage and immune to charmed, exhaustion, grappled, paralyzed, petrified, poisoned, prone, and restrained conditions.

Abilities

 * Incorporeal Movement: The wraith can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
 * Sunlight Sensitivity: While in sunlight, the wraith has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
 * Life Drain: Wraith makes a melee weapon attack against one creature with +6 to hit and reach of 5 ft. If hits, deals 21 (4d8 + 3) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
 * Create Specter: The wraith targets a humanoid within 10 feet of it that has been dead for no longer than 1 minute and died violently. The target's spirit rises as a specter in the space of its corpse or in the nearest unoccupied space. The specter is under the wraith's control. The wraith can have no more than seven specters under its control at one time.

Wraith warriors
Sword wraiths are undead who had aspiration for glory but fell with no honor in battles in the times of long past, and now they haunt their ancient battleground. They are often resistant to necrotic damage and nonmagical damage from bludgeoning, piercing, and slashing attacks. They are also immune to poison damage and immune to exhaustion, frightened, poisoned, and unconscious conditions.

Known wraiths

 * On the Island of Viscan, two wraith inhabited cages in Opash's lab. During the Vox Machina's fight with Raishan, they attacked the party, but were ultimately scattered by the green dragon's Meteor Swarm.
 * In the Ruins of Thomara, Viskorad summoned two wraiths, attempting to delay Vox Machiana.
 * Lord Cioro, an individual of high station and importance in Ghor Veles. Lord's body found its final resting place in the part of the city that was caught by Vokodo's teleportation magic and transported to the island of Rumblecusp. The wraith-like entity, leading a swarm of ghosts, the Mighty Nein have encountered near the ruins of Ghor Veles might have been the Lord himself.
 * As a part of the "Shard of the Shadebarrow", an adventure hook for both Tal'Dorei Campaign Setting and Tal'Dorei Campaign Setting Reborn, a wraith claiming to be the spirit of Avandros was active in the Shadebarrow, where it had animated an army of ghasts and ghouls and conquered the Silvercut Crossroads.

Neatherdeep

 * Eight sword wraith warriors guard a watchtower of Cael Morrow under the leadership of a sword wraith commander named Beltreath. They are former defenders of the city who fell during the battle against Gruumsh and, as of 836 PD, protecting the ghost of Hadarai.
 * In the Grottoes of Regret in the Netherdeep, when entering a Haunted Ashes locations, an individual is forced to relive Alyxian's memories of his allied soldiers, who aid him in the battle against Torog, pillaging the village, they previously saved. In his memories, the soldiers take form of four sword wraith warriors led by a sword wraith commander named Kalagothe.
 * In another Grotto of Regret, Alyxian remembers the battle at the Betrayer's Rise. The hordes of his enemy, Torog, take form of sword wraiths.