Death Dog

A death dog as ally is a stretch you make work by leaning on the disease as an asset. A necromancer's pet, a plague-cult guard animal, a gnoll-tribe-bonded warhound that has switched sides because the party freed it from a cage. Whoever owns it owns a biological weapon, and the moral weight of using it is the scene.

The dog is Intelligence 3, Charisma 6, and speaks no languages, so it does not parley or scout meaningfully. Treat it as an animal companion stat-block on the party's side, with Multiattack two Bites at +4 for 4 piercing each plus the DC 12 Con save for the wasting Poison. Set it on the most expendable enemy in the encounter and watch the rest of the table react. A bandit lieutenant who breaks free will limp away dying over the next week, which is a story consequence the party has to live with.

Out of combat, the dog is unsettling to townsfolk. Innkeepers refuse the party rooms. Children cry. A paladin in the group will eventually demand the dog be put down, and the answer to that conversation should not come from you.

If the dog is killed, do not give the party a replacement. The point was the moral weight, and replacing it with a normal mastiff after the fact undermines the whole arrangement.

Have the dog cock both heads at someone the party trusts, the round before that person betrays them. Death dogs see things.

A death dog is a horror-movie ambush in beast form. Two heads, one body, 39 HP, AC 12, and a bite that gives the Poisoned condition with a hidden long-term cost. The damage on the bite is 4 piercing per hit, which sounds harmless until you read the saving throw text. Failing the DC 12 Constitution save Poisons the target and locks their Hit Point maximum from recovering on a long rest until they make a successful save on the daily repeat. Subsequent failures shave 1d10 off the maximum. This is wasting disease, and it is the entire point of the encounter.

Run death dogs in pairs or small packs of three. The Multiattack is two Bites, so a single dog forces two saves a turn against an unarmored target. Pack two of them on the rogue and you'll get four save rolls before the cleric's initiative comes up. Stealth +4, Perception +5, and Darkvision 120 ft. say everything about how this fight should start: the party hears nothing, then a dog is on someone's neck. Open at night, in dense brush or scrub, with Passive Perception 15 deciding who notices.

Death dogs are also Charmed-, Frightened-, and Stunned-immune, so most low-level control spells just bounce. The party will need to focus damage. They drop fast once attention turns to them, with 39 HP and AC 12, but the disease is already in the bloodstream by then. The encounter's danger isn't the fight; it's the next morning when the fighter finds his HP maximum is 47 instead of 52 and nobody can lift the curse without a Lesser Restoration.

If the party brings overwhelming force, the surviving dog flees toward whatever ruined shrine or gnoll camp it came from. It will try again at the next watch.

Have one PC make their first long rest after the bite and quietly tell them their HP maximum hasn't fully refreshed. Don't explain why. Let the cleric figure it out.