Chain Devil

A chain devil on the party's side is a contract. A warlock with an Infernal patron has been lent this thing for one night, or a paladin made a desperate bargain to free a soul, or a noble's debt to Hell is being paid by sending its agent to help the party first. The devil shows up, asks who needs binding, and waits with chains coiled on the floor. It does not chat. It does not protect. It binds whoever it is told to bind, and the moment the contracted job is done, it leaves.

Use it for one specific scene. The party needs to capture a fleeing necromancer alive. The chain devil opens with Conjure Infernal Chain at DC 15, drags the target 30 feet, then Multiattacks with two Chain attacks to grapple and Restrain. With a +7 to hit and an escape DC of 14, almost no one breaks free without help. The party finishes the unconscious target however they like, and the chain devil collects its body for the contract. If a PC tells it to release the captive, it tilts its head and says "that was not the agreement."

Two warnings worth telegraphing before the scene. Unnerving Gaze still triggers on PCs who start their turn within 30 feet, so the party's allies make the same Frightened save the enemies do. The devil does not turn it off. And telepathy 120 ft. means whoever signed the contract hears the devil's voice in their head all night, narrating the work in flat infernal calm.

Have the devil ask a single offhand question before it leaves. Something specific about a PC's family, or a detail it should not know. Hell remembers.

A chain devil is the torture-room middle manager of the Nine Hells. It does not lead infernal armies and it does not negotiate. Someone (a cult, a higher devil, a doomed warlock) summoned this thing to a fixed location to break specific people, and it has been waiting in the dark for the party to walk in. Set the scene before initiative: hanging chains, drag marks, the smell of cooked iron. The chain devil announces itself by rattling.

Open with Conjure Infernal Chain on the squishiest visible target. DC 15 Dex save, 9 fire damage on a fail, Restrained until the end of its next turn, and the devil yanks them up to 30 feet straight toward itself if they're Large or smaller. That's the whole opening play: separate the cleric from the fighter and pull them into the kill box. Then close with two Chain attacks at +7 (11 average slashing each) and grapple them with both Chains, leaving them Restrained against the devil's body until they spend an action to escape DC 14. A grappled wizard cannot move and cannot cast Misty Step without losing the chain.

Unnerving Gaze is the trap on every PC turn. Anyone starting their turn within 30 feet who can see the devil makes a DC 15 Wis save or is Frightened until the end of that turn. Frightened means disadvantage on attacks against it and no moving closer, so the party's melee plan stalls out. Save the rare reaction for the player who looks ready to commit. Magic Resistance gives advantage on every save against spells, so Hold Monster and Banishment are coin flips at best. Resistance to non-magical bludgeoning, piercing, and slashing means a level 5 fighter with no magic weapon does half damage all night.

The chain devil does not flee. Diabolical Restoration means death just sends it home, so it fights to zero with no retreat instinct. When it drops, have its body collapse into smoking links of chain that crawl back toward the summoning circle. Someone in the world just got an alarm.