Bearded Devil

A bearded devil on the party's side means a contract, almost always one the party is going to regret. The cleanest setup is an Infernal Bargain a warlock or rogue made years ago that's now coming due as a favor in the other direction; another is a devil "lent" to the party by a higher-ranked devil who wants something specific destroyed and doesn't care who does it.

Run the devil as a hostile coworker. It speaks Infernal and has telepathy 120 ft., so it will communicate with the party only when it suits it, and otherwise stay silent. It does not take suggestions, only orders, and only orders that fit the letter of the contract. Anything outside that is met with "that was not part of the arrangement," in writing if the GM wants to be cruel about it.

In combat, the devil prioritizes whoever the contract identifies as the target. Beard goes on the target's healer first to deny revives. Infernal Glaive opens wounds on whoever else is closest. The devil does not protect party members, will not take the Help action, and absolutely will not heal anyone (it can't anyway). If a PC goes down within reach, the devil might step over the body to get to the contracted target.

When the job is done, the devil vanishes back to the Nine Hells and leaves a small parchment receipt. The party should feel like they've borrowed something they shouldn't have.

Write the receipt before the session. One sentence. Sign it in red.

Bearded devils are infernal shock infantry that punish healing more than they punish hit points. Run them in pairs or trios; one alone is a speed bump, but three with a caster behind them turn a midgame fight into a logistics problem.

Open by closing distance. They walk 30 ft. and have no ranged option, so the first round is just getting into reach. Once there, Multiattack opens with one Beard and one Infernal Glaive. The Beard hit applies the Poisoned condition until the start of the devil's next turn, and while poisoned the target can't regain Hit Points. That's the whole gameplan: tag the cleric or the paladin with the beard, then swing the glaive at someone else. The party heals at half effectiveness for as long as a beard is in play.

Spread the Infernal Glaives. Every glaive hit on a clean target forces a DC 12 Con save or that target takes 1d10 at the start of each of its turns until the wound is closed. Closing a wound burns an action on a DC 12 Medicine check or a healing spell, both of which are exactly what the Beard is designed to deny. Two devils can stack three or four open wounds across the party in a single round. Magic Resistance gives advantage on saves against spells, so don't let a wizard delete a devil with one Banishment; play the advantage and stay on your feet.

Devils are immune to Fire and Poison and resistant to Cold, so the party's reliable AOE picks usually fall flat. Force, radiant, and silver weapons cut through. If a paladin announces Divine Smite, that's the devil to drop. They have no Frightened immunity from Magic Resistance directly but they are immune to the Frightened condition itself, so save your Fear effects for something else.

Bearded devils don't surrender, but they do report. If reduced to single digits and a stronger devil is within telepathic range (120 ft.), the bearded devil retreats one round to deliver intelligence before dying.

Have one devil grab the party's healer by the beard and drag them ten feet away from the rest of the line. The visual is unambiguous and the party will feel the loss before the math hits.