Zombie

A zombie on the party's side belongs to a necromancer PC, a desperate cleric, or a one-time consequence of a relic the party wishes they had not picked up. The framing matters because a thrall raised by a friendly necromancer is fundamentally different from one a paladin tolerates because the alternative was leaving a friend dead in the road. Pick which version of the leash the party has and play the moral cost out loud.

In combat the zombie is a CR 1/4 mook with Slam at +3 and 15 HP, which means it exists to soak one attack and contribute one swing per round at modest damage. Point it at the largest enemy and let it walk forward. Undead Fortitude on the party's side is a real perk: the necromancer has a chance to keep the zombie up across rounds without spending another spell slot, and that chance scales inversely with how hard the zombie is being hit. Tell the player to ration their hits across multiple zombies rather than letting one absorb a fireball.

Zombies are immune to Poison damage, the Poisoned condition, and Exhaustion, which makes them ideal for plague tunnels, swamp dives, and any environment that would chew through a living hireling. Use that. A zombie that walks ahead through a poisoned cloud is a useful body, and the party will accept the use even if they squint at it.

Decide before the session what makes the zombie stop being an ally. A radiant aura, a temple ground, a particular phrase from the dead person it used to be. The leash should be thinner than the players think.

A zombie is the encounter you use to teach players that "down" is not the same as "dead." With 15 HP and AC 8 it is trivially easy to drop, but Undead Fortitude means a Con save (DC 5 plus the damage taken) at 0 HP brings it back to 1 unless the damage was Radiant or a critical hit. The first time a player swings their longsword, drops a zombie, and then watches it stand up next round is the moment this monster pays for itself.

Run zombies in groups. A single one is a speed bump. Six of them, walking 20 ft. a turn through a fog or a graveyard, are a horror set piece. Slam is +3 to hit for 5 piercing or bludgeoning, which means each zombie hits about half the time and does light but consistent damage. Have them target whoever the party is trying to protect: the wounded NPC, the unconscious paladin, the wizard who just dropped Sleep on three of them. Zombies do not flank, do not hold actions, and do not assess threats. They walk toward the most vulnerable target in line of sight and slam.

Use the Undead Fortitude save as a pacing tool. The cleric who knows Sacred Flame should feel like the only person in the room making real progress. The radiant immunity loophole, plus crits, plus area Radiant from a Daylight or a Spirit Guardians, are the answer keys you want the party to discover. If a zombie keeps standing back up two or three times, telegraph the rule: "the longsword wound stitches itself shut, and it stands again." The fight should feel grim, not unfair.

Have one zombie in the back wear what looks like a familiar cloak. Let a player roll Insight before they realise it is the missing miller from town two. The horror of zombies is who they used to be.