Imp

An allied imp is a warlock's familiar or a contract spirit a party member has bound through a Find Familiar variant. The imp is Lawful Evil and will say so, openly. It serves the contract, not the warlock, and it will report on its master to whoever holds the contract above. That treachery is built in. Make the warlock player choose whether to hide it from the rest of the party or be honest about what they signed.

In play, the imp is an extraordinary scout and a poor combatant. Invisibility at-will (concentration via the spell, ends if it attacks), fly 40, Shape-Shift to a rat or raven for daytime infiltration. Darkvision 120 ft. means it owns the night, and Stealth +5 plus Insight +3 mean it returns with both the building plan and a read on the room's mood. The party should be using it for everything they cannot do themselves: deliver a vial, open a window, eavesdrop on a meeting from inside a chimney.

In combat, the imp's Sting at +5 plus the 7 poison is a useful finisher on Bloodied targets, but its 21 HP and AC 13 fold to one solid hit. Keep it invisible. Magic Resistance gives Advantage on saves against the enemy spellcaster's first Hold Person, which is more useful than any damage the imp will roll. Once the imp judges the encounter lost, it Shape-Shifts and leaves. The warlock player has to live with that.

Have the imp ask, every time the party makes a moral choice, whether they are sure. Not loudly. Just once, in Infernal, into the warlock's ear.

An imp is a familiar with a contract and a knife. It almost never fights as the headline encounter; it scouts, betrays, poisons, and reports back to a warlock or devil patron the party has not met yet. The whole stat block is built for harassment from invisibility: at-will Invisibility cast on itself, fly 40, three Shape-Shift forms (rat, raven, spider), Magic Resistance for the inevitable Detect Magic, and a Sting at +5 for 6 piercing plus 7 poison.

Open the encounter with the imp already in the room as a spider on the wall or a raven on the rafter. Insight +3 and Deception +4 mean it sizes the party up over hours of observation before it ever attacks. The first action is usually to vanish (Invisibility) and report. If combat does start, the imp opens from invisibility with Sting on the squishiest available target, takes the hit and the breaking of invisibility, then flies up to 40 ft. and casts Invisibility again next round. With 21 HP, AC 13, Cold resistance and Fire/Poison immunity, it usually survives one round of being seen.

Spread the poison. The imp wants to land Sting on as many PCs as possible, not pile damage on one. Two failed bites can drop a level 3 wizard; three across the party softens everyone for what comes next. Magic Resistance gives Advantage against Hold Person, Banishment, and the rest, so casters quickly figure out the imp is best handled with mundane attacks they can't actually land. That's the joke. Once Bloodied, the imp Shape-Shifts into a rat and squirrels into a wall to report. It does not die for the patron unless ordered to.

Have the imp speak Common with the patron's voice. One sneering line in the bard's own cadence, then Invisibility. The party should know they're being watched by something amused.