Vampire Familiar

A vampire familiar on the party's side is a campaign turn, not a starting condition. The framings that work: the party has killed the master and the familiar is now free of the Charm immunity clause and trying to figure out who it is, the party has struck a deal with a different vampire whose familiar they have borrowed for one job, or a player character is the master (warlock with a vampire patron, necromancer of some kind) and the familiar is the patron's gift. The first framing is the most interesting and the hardest to play. A freed familiar is a recovering addict.

In combat the familiar is a precision skirmisher. Two Umbral Daggers per turn at +5 for 5 piercing plus 7 necrotic each, with the knockout-to-Paralyzed rider on the killing blow, makes this a great tool for taking enemies alive. Direct it at the enemy spellcaster the party wants to interrogate, and the familiar puts them at 0 HP, Stable, and Paralyzed for an hour. The party gets a captive without having to dance around lethal damage rolls. Climb 30 ft. plus Deathless Agility (Dash or Disengage as a bonus action) means the familiar gets to its target across most terrain, then disengages before the next turn.

The familiar refuses three things. It will not enter consecrated ground without a real reason. It will not cross running water in moonlight (a Drinker-of-the-Old-World quirk; if your setting doesn't carry vampire weakness lore, drop this one). And it will not speak about its time under the master, ever. Players who push will get a long silence and the familiar will not be available the next time they ask. Treat the familiar's history as a wound, not a backstory dump.

Have the familiar bring back a single object from each scout mission and place it on the table without explaining. A button, a key, a child's drawing. The accumulating pile is the character.

A vampire familiar is a thrall, not a vampire. CR 3, AC 15, 65 HP, walk 30 ft., climb 30 ft., resistant to Necrotic, immune to Charm from anything except its master, and equipped with ten daggers and a Multiattack of two Umbral Dagger strikes per turn at +5 for 5 piercing plus 7 necrotic each. The signature line is in the dagger text: a target reduced to 0 HP by the Umbral Dagger becomes Stable but Poisoned for 1 hour, and while Poisoned that target is also Paralyzed. The familiar exists to capture, not kill. Its master is collecting bodies for later.

Run the familiar as a recurring scout. Vampiric Connection lets the master see and hear through the familiar at any distance on the same plane, so this NPC is the master's eyes and ears in town long before the party realizes there is a vampire problem. The opener should be a social scene: a too-helpful guide at the inn, a quiet servant in a noble's house, a child who keeps showing up at the edges of the party's investigations. Stealth +7 and Persuasion +4 carry the early scenes. When the fight starts, the familiar climbs (30 ft. climb, no roll needed on most surfaces) to a height the melee party can't reach, then throws daggers at 20/60 ft. range. Two daggers per turn, and the familiar carries ten. Bonus action Deathless Agility for Dash or Disengage means it never gets pinned.

The right target is whoever has the lowest HP at the end of the round. The familiar wants kills (or rather, knockouts, since the dagger leaves them Stable and Paralyzed). Once a PC is down via the Umbral Dagger, the familiar tries to drag the body away. This is the encounter the party should fear: not the death, but the disappearance. The PC wakes up an hour later in the master's cellar, Paralyzed, with no spell components and no weapons. That is the next adventure.

The familiar will not last-stand. Once it is below half HP, it disengages, climbs out a window, and reports to the master. Let it escape. The party that kills the familiar in scene one has cut the master's eyes off and learned nothing. The party that lets it slip away is now being watched.