Hell Hound

A hell hound on the party's side is rare and means somebody owes somebody. The most plausible setup is a tiefling PC whose patron loans a brace of hounds for a single hunt, or a paladin who freed the hounds from a cruel devil and now they follow her until the debt is paid. The hounds will not pretend to be dogs. They smell of sulfur, they leave scorch marks where they sleep, and innkeepers will not let them inside.

In combat, point them at one target and they handle it. Pack Tactics with two hounds turns Bite into a high-Advantage threat against any single PC enemy, and Fire Breath in a 15-foot cone covers the gap when the party can't get a frontline in. Speed 50 means they screen for the party's casters by getting between them and any approaching melee. Don't bother coordinating tactically with the players; the hounds understand Infernal commands only, and only the simplest ones.

Out of combat, they hunt and they guard. Perception +5 and passive 15 makes them better night sentries than most NPCs, and Darkvision 60 ft. covers the rest. They will not retrieve, fetch, or play. They will tear apart anything that approaches the camp without the right scent.

When the loan ends or the debt is paid, the hounds leave together at dawn and the party never sees them again. If a player insists on adopting one, the hound stays for a session, kills something the player liked, and then leaves on its own. Hell hounds are not pets. Make that point once and the party will remember.

Hell hounds are pack animals that breathe fire. One hell hound is a footnote; three or four hunting together with Pack Tactics on every Bite is a real fight even for a mid-level party. Run them in a pack of at least two, ideally with a handler giving them a target.

Open with Fire Breath if it's available and the party is bunched. DC 12 Dex save in a 15-foot cone for 5d6 fire is a soft hit on its own, but two hounds breathing on the same cluster is 10d6 across two saves, and recharge 5-6 means it's likely back in a round. After the breath, close and start biting. Bite is +5 for 1d8+3 piercing plus 1d6 fire, two attacks per round, and Pack Tactics gives Advantage as soon as one hound flanks a target. Speed 50 means the hounds get into position the same turn they start the encounter. The party's wizard is in melee whether she planned to be or not.

Target priority is whatever the handler points at. Hell hounds understand Infernal but can't speak, so a devil, cult leader, or fire giant nearby gives them orders and they execute. Without a handler, they pick on smell and movement, which means the rogue who tried to sneak past gets pulled down first. Fire immunity means a fireball is wasted; a cleric who burns a third-level slot on radiant or thunder is your real problem.

Hell hounds don't retreat from a fight they're winning, but they will break off if the handler dies and the pack is below half. Have one survivor flee back to its master. The party will follow the trail and find the next encounter waiting, because the hounds got there first. That's the chain that turns a single pack into a campaign thread.