Wraith

A wraith on the party's side is an uncomfortable bargain, and you should make the discomfort the point. Cleanest framings: a vengeful spirit who shares the party's target, a warlock patron lending a wraith for one job, or a temple ritual that briefly chains the wraith to the cleric's authority. The wraith does not become good. It becomes pointed.

Use it for one job in one night. The fly speed of 60 ft. with hover and Incorporeal Movement makes the wraith a perfect infiltrator: it floats through every wall, ignores locks and traps that target physical bodies, and reports back via the warlock's telepathy or the cleric's command. Daylight is poison; the bargain works at night, indoors, or in a tomb, and never on the party's terms in an open field. Sunlight Sensitivity is not flavor.

In combat, point the wraith at one target and let it do what it does. Life Drain at +6 for 21 necrotic plus max HP reduction is a fight-ending ability against a single named enemy. Tell the player whose patron sent the wraith: one command per round, and that command is a target. The wraith will not protect anyone, and it will absolutely not refuse to feed if a wounded ally is closer than the assigned target.

Create Specter is the line you do not let the party cross. If the wraith asks to raise a fresh corpse and the party agrees, the campaign now includes a controlled specter that belongs to a hostile undead, and that is a problem the party will inherit later. Make them refuse out loud or pay for it.

When the job is done, the wraith leaves before sunrise without a word. If the party tries to renegotiate, it Life Drains the closest one and goes through the wall.

A wraith is a hate-shaped hole that floats through walls and feeds on living souls. The mechanical signature is Life Drain (+6 to hit, 21 necrotic average, and the target's hit point maximum drops by the damage taken until a long rest with a Greater Restoration on top), plus Incorporeal Movement that makes terrain irrelevant. Run it as a haunting first and a fight second.

Stage the encounter at night and indoors. Sunlight Sensitivity gives the wraith disadvantage on attacks and ability checks in sunlight, so a daytime fight in the open is the wraith losing. The wraith knows this and picks crypts, dungeon halls, and the inside of a manor at three in the morning. Hover 60 ft. with a 5-foot walk means it never touches the floor; it drifts through walls and ends its turn in safe space, because Incorporeal Movement costs 1d10 force damage if it ends inside an object.

Open with Life Drain on the squishiest target the wraith can reach. Spellcasters and ranged characters lose hit point maximum on every hit, which means a cleric who took two strikes is functionally a third-level cleric for the rest of the day. Resistance to bludgeoning, piercing, and slashing (plus acid, cold, fire) and immunity to necrotic and poison turn most martial damage to half. Force, radiant, thunder, and lightning are the answers; if the party brought none of those, the fight is going to last.

Create Specter is the campaign threat. The wraith targets a Humanoid corpse dead less than one minute and raises a specter under its control, up to seven at a time. Use this on a slain NPC the party tried to save, or once per campaign on a freshly killed party member as a horror beat. The wraith does not need to use this in combat; it uses it in the scene after, while the party stands over the body deciding whether to rest.

Have the wraith say the player's character name once, in the voice of someone that PC has already lost. The fight is half mechanics and half temperature.