Specter

A specter does not ally. It has Wisdom 10 and Charisma 11, no diplomacy worth speaking of, and one drive, which is to drain the living. The framing has to be control: a necromancer who has bound it, a priest of a death god who has yoked it to a single grave, or a relic that contains a specter and releases it on a command word for one round. The party is allied with whoever holds the leash. The specter is a weapon that hates them slightly less than it hates the enemy in front of it.

Use it for one job in one room. Send the specter through a wall ahead of the party, let it Life Drain the spellcaster on the other side, and recall it before it can decide to come back through the wall the wrong way. Incorporeal Movement makes the specter a perfect assassin against any target sealed behind a door, since the door is not the obstacle the enemy thought it was. The maximum-HP reduction effect is permanent until a long rest, so the enemy spellcaster who survives the round is still down 7 max HP for the rest of the encounter.

The specter does not coordinate. It will Life Drain whoever is closest to it the moment the binding wavers, and that closest target is sometimes a party member. The handler should keep the binding taut, and the party should keep their distance. Sunlight Sensitivity means the trick fails in daylight, so this is a night-and-tomb tool only.

When the bound task ends, the handler dismisses the specter back into whatever vessel holds it. If the binding fails mid-task, the specter turns on the nearest warm thing and the party has about ten seconds to remember which damage types actually work against it.

Have the handler trace the binding circle in chalk and salt before the door opens. Players take the threat seriously when the spell's cost is visible.

A specter is a ghost without the manners. It died violently and remembers almost nothing of its life except the urge to make the living share what happened. Run it as a swarm encounter, not a duel. One specter at CR 1 is a nuisance for a level 3 party, but four specters in the same crypt is a real fight, since Life Drain stacks across attacks and the maximum-HP reduction does not heal until a long rest.

Open from inside a wall. Incorporeal Movement plus a hover speed of 50 ft. lets the specter wait inside the masonry until the rogue passes, then drift out and Life Drain at +4 for 7 necrotic. The damage is moderate. The hit-point-maximum reduction is the trick: the rogue's max HP drops by 7, and now the cleric's healing only restores them to that lower ceiling. Two or three Life Drains on the same target across a fight and that PC is a third of the way to dead even after a rest. Spread the hits or concentrate them, depending on whether you want to scare the party or kill someone.

The specter has 22 HP and AC 12, but resists every common physical and elemental damage type, so the fighter's longsword does half. It is immune to Necrotic and Poison, and immune to most disabling conditions, including Paralyzed, Petrified, Grappled, and Restrained. Force, Radiant, and Psychic are the weaknesses to reward. Sunlight Sensitivity is the player-side counter: in direct sun the specter has Disadvantage on attack rolls and ability checks, so the encounter has a clean answer if the party fights it out of the crypt. The party that drags the specter into the morning has earned the easy win.

Specters do not retreat, parley, or recognize the players. Int 10, Wis 10, Cha 11 mean they have just enough mind to hate. They fight to 0 HP and dissipate, and any Humanoid they killed in the encounter rises as a specter in 24 hours unless someone burns the body and salts the ashes. Mention this. Watch the cleric look at the bodies.

Describe Life Drain as a cold seeping into the bones, then quietly subtract the number from the player's max HP on their sheet. The mechanic is the horror.