Vampire Spawn

A vampire spawn on the party's side is the rogue's secret, the necromancer's leverage, or the warlock's accidental child. Spawn are bound to a master, so a friendly spawn is usually a spawn whose master is dead, hidden, or temporarily aligned with the party. The fiction work is the master question. Resolve it in the first scene and the spawn becomes a useable NPC. Leave it ambiguous and the party will spend three sessions worrying about who the spawn is reporting to.

In combat, the spawn opens from the ceiling using Spider Climb and Stealth +6, drops on the largest enemy, and grapples with a Claw. Bite at DC 14 Con on the grappled target heals the spawn for the necrotic dealt, which means the spawn is a self-sustaining anchor in a long fight. Deathless Agility as a bonus action covers the disengage when the party calls for a tactical reposition. Necrotic resistance lets the cleric blast through the spawn's space with Inflict Wounds without much risk to the spawn itself.

The cost of the alliance is the weaknesses. Forbiddance means the spawn cannot enter homes uninvited, which the village will notice. Running water (20 acid at end of turn) means river crossings are a planning problem. Sunlight (20 radiant at start of turn, Disadvantage on attacks and checks) means daytime travel is a tarp-and-coffin operation. Stake to the Heart is the kill switch any farmer with a pitchfork can execute on a sleeping spawn, and the players should know that. The relationship is the campaign.

Have the spawn refuse the bite even when offered. The horror is what it would do, not what it does. Save the actual feed for the moment a player betrays it.

A vampire spawn is the encounter you put between the party and an actual vampire, and it is mean enough on its own to anchor a tier-2 fight. CR 5, 90 HP, AC 16, Spider Climb up the walls, and a Bite that drains Hit Point maximum on a failed DC 14 Con save. Run it as a horror first and a damage stat block second. The party should fear the bite more than the claws, even though the claws hit harder per attack.

Open with positioning, not Multiattack. Spider Climb plus Stealth +6 means the spawn is on the ceiling above the party before initiative if you set up the room well. Round one, drop on the squishiest target with two Claws (+6, 2d4+3, average 8 each) and force a DC 13 Strength check to escape one Grapple. The grapple sets up Bite, which is the whole point: DC 14 Con save for 1d4+3 piercing plus 3d6 necrotic on a fail, with the target's HP maximum reduced by the necrotic taken and the spawn healing the same amount. That's an average 10 swing per Bite, repeated every turn the grapple holds. The spawn becomes harder to kill the longer the fight runs.

Use Deathless Agility intelligently. The bonus action Dash or Disengage means the spawn never provokes opportunity attacks and never gets stuck in a bad position. After a Multiattack on the wizard, Disengage and climb to the ceiling for a turn, then drop on the cleric next round. Necrotic resistance keeps the spawn alive against an Inflict Wounds, but radiant damage and silvered weapons hurt as expected. The party will figure out that sunlight is a button, but check the room: if the fight is in a crypt at midnight, the button is unavailable.

Vampire spawn do not retreat. They are bound to a master and the master's orders trump self-preservation. A Bloodied spawn will throw itself at whoever is healing the party, since dropping the cleric means a slow win on attrition. The fight ends when the spawn drops or the party does. Plan the room with that in mind.

Cue the spider-climb with a wet sound from the ceiling before initiative. The party should look up exactly once, and then it should be too late.