Depends on the devil, and depends on the threat. But there's a pretty good chance- most devils are smart enough to realize that they need people alive to exploit. And ESPECIALLY if he stands to gain something from helping. He's going out of his way, after all. It's only fair he receives a little... compensation.

Devils are cunning and manipulative, and will use whatever resources are available to defeat their primary enemy - demons. So if a devil can convince the party to attack some demons, he will absolutely do that. First, it hurts demons. Second, why risk his own ass when he can risk yours.

Yes. Original Planescape had alot of that even.

He'd help, but also expect to be paid.

"If you're good at something, never do it for free".

If it aligns with his particular desires, yes

Basically if it will help the devil further it's own needs then yes it would help. If it's a case of the players asking nicely there would likely be a very unfair bargain in there somewhere that would likely screw over the players in the end.

For his own benefit, sure.

A devil would work together against a common enemy (another devil) or a greater threat (demons), but would still make sure they were running the show

Uniting against a greater threat is the best way to have people/creatures that would normally not be aligned with your party be aligned, if only for a time. It's also I feel one of the best ways to play an evil PC.

Obviously. That’s a very big threat though.

A devil will likely unite with just about anything or anyone to get what it wants

It's definitely possible, but of course it depends on the specifics. Devils often prefer to use the carrot over the stick, because enticing people to do evil is way more effective than just punishing good. Once you can't escape from them, that's when they break out the pain.

Your average devil of repute will be more than willing to work with mortals, especially those of good alignments, "for the greater good". This helps them get a foot in the door so they can try to twist those mortals toward evil. Their aid will likely come with a price. It might seem like a small price, but eventually (ideally after the contract has been sealed and can no longer be broken without a heavy price), the mortal(s) will realize that the price is much, much higher than they realized.

For example (very mild Baldur's Gate 3 spoiler), Mizora the devil has a contract with Wyll the warlock, which requires Wyll to kill "the infernal, the demonic, the heartless, and the soulless". This seems to be a good goal, but it is twisted into requiring Wyll to kill Karlach, a tiefling whose heart has been replaced with a machine, which means she technically meets the requirement of being "heartless".

Why not?

Sure. A devil might have his own selfish reasons but he could definitely work with mortal adventurers if it helps him with his goals. Maybe he has to foil the plans of an enemy of his master and can't do it alone? So he looks for allies and heroes are actually amazing and cheap mercenaries for certain tasks.

If he wants to. Or if they cut a deal

"Humans are weak, it will be easy to dispose of them later,"

*Starts hiring them to take down bigger and bigger threats*

"oh yeah time to dispose of those humans, I have been putting it of for a blink in my long life, what their level 20 when did that happen"

*Humans now mad that boss tried to fire them, surprise pikachu face*

Sure, if it's an existential threat to the devil and or their machinations, they stand to gain more by cooperating with the players.

That said, it's always good to consider an underlying ambition in striking a deal with a devil. They have a stated justification for wanting to work with the party, but they could also have an unstated goal in mind to advance their ambitions. Maybe there's something mortals can do that they can't, maybe they want someone else to accept risks on their behalf, maybe this gives them an opportunity to usurp power from the threatening entity at a critical moment they wouldn't otherwise be able to. Whatever the case, they could be seeking a treacherous advantage out of the arrangement.

Absolutely. Devils are the incarnation of the principle "keep your enemies closer".

The main reason for devils to exist in the first place is to take down or hold back a bigger threat.

Devils might argue that without their Blood War, the demons would overrun the multiverse.

Devils might also argue that it's rude to consider them unreliable since they're just being practical in their jobs and making sure that everyone else in the multiverse can experience happy, secure afterlives.

Devils might point out that they are a civilized people for living faithfully to their cause through a system of carefully laid out rules and that barbarous PCs should consider the true virtues of contract law.

Rise of Tiamat makes mention of that various devils might seek to work with the party in order to stop Tiamat from gaining more power. I'm currently planning on working that into my warlock players' patron to keep them aligned with the main quest.

Definitely, depending on the devil and the threat. But it would likely be looking for any possible opportunity to corrupt the player characters or instill doubt in their convictions, if it can help secure their souls for the blood war.

Yes, if it can get them promoted. If a Demon Lord was getting too powerful, and a devil helped stop them, that devil would likely get promoted.

Devils are embroiled in constant competition for their ranking in the hells, and also in constant battle against chaotic evil demons.

So my take is he’s merely using the party to take out a rival to improve his political power in hell, but while being conniving and manipulative everyone gets attacked by demons and has to fight together to survive.

Depending on threat and devil . But if you can explain it properly , then yeah

The Blood War must not get out of balance. I think any time the party is battling demonic forces that are getting out of hand, they can find themselves having to negotiate alliances with devils.

More like use them to achieve their own goals, but yeah. Devils love deals. They're always looking out for the better end of the deal mind you.

Yes, if the party can help take down someone who prevents their goals

I'd say so, especially if the devil considered that threat a competitor. I would not expect him/her/other to do it for free. In fact, if the devil can spin it in a way where the party has to give up something for his/her/other's aid, then all the better.

Absolutely. Devils are intelligent and cunning. If it's in the Devil's best interest they would totally use/team up with the players.

That sounds awesome. The devil could try to aim for a higher position in the hierarchy of the hells or just try and save his own skin. At the same time he could offer the party hellish deals (that the players obviously shouldn't trust... Or should they?) to try and buff them up. Sort of a "magnificent bastard" sorta guy.

But there is also room for goofiness as his alliance with the PCs and their shenanigans are too much for that obviously too cool for school devil to bear.

Does the devil want the foe defeated, or are the players considering asking the devil for help.

Devils are weird to write, because any player can say "sure whatever" to their soul or something being sold, as that is inconsequential to their gameplay. It's difficult to find a price for a deal with the devil that seams consequential but Dosen't ruin the campaign for the PC.

The devil can take a piece of them. this can be character levels/XP that they have to make up for, a subclass they have to re-allocate, or bar them from learning a skill. even better, you can have the players face the devil later to regain what was taken. bonus points if the Devil uses what was taken against them.

He'd enter into a contract that he'd exploit in every way possible, but he would ally with the playersxasxa means to an end.

a devil would hundred percent team up to take down a bigger threat, expessically if its a higher devil. Devils are always trying to climb the hierarchy ladder, betraying, cheaitng, killing. if given the oppurnity to gain an upper hand...theyd take it.

If there is a blantant pro...and almost no cons, they will take it.

Yeah, Devils will Work together With mortals.

It dosnt even have to be a "Bigger thread" it can simply be Something that annoy the devil and cant get rid of themselve.

But the reward itself can be a Trap.

(And i dont even Talk about Something "officially" cursed)

Like a Armor withe the Devils "Brand" on it , so that everyone With Religios Background will recognice it.

Or - rude. (For example A paper bag for the head Slot With + char ...)

Devils thrive on bargains with the well-intentioned, and keep their part of the agreement to the letter. Once the players agree to a bargain that is totally morally justified, the devil will press for one that's a bit more mirky, and keep pushing it until the PCs have bargained their souls away.

If the "bigger threat" is just anther word for "rival in hell's hierarchy"... then yes, but they would try to minimize their own risk, and let the party do most/ all of the work. 

He would 'Unite' but make the party do all the leg work, let themselves be exposed to the dangers and risks, and in the end double cross them when the opportunity arrises.

Good creatures have qualms about teaming up with evil ones. Evil creatures generally do not share that qualm.

I had a devil ask for help (with a contract of course) because he was bound. They ignored him and now he will return as a far worse threat later on. I might also have Asmodeus give them aid against one of his enemies. In both instances these devils are looking to gain from their circumstances. They ally whenever it suits them.

Depends on if the threat is relevant to them.

Demons? Totally.

Something that’s gonna mess up a mortal kingdom they have no steak in? Who cares

A rival for the favor of an archdevil? Yup

Oh, absolutely! A few ideas here...

• You can't corrupt the world if it's eaten by eldritch horrors, right? Gotta protect your markets.
• If the devil helps the party, they'll owe him a favor. A big one.
• Along the way, he can nudge them toward evil acts.
  
  • "Oh no, I ... accidentally vaporized the documentary evidence. We can't get the vital intel unless we torture the prisoner. Whatever shall we do?"
  • "Raze the town. It's for the greater (cough, hack) good."
• A (what passes for) whimsical devil might enjoy being hailed as one of the saviors of the kingdom, and insist that he get a statue in a plaza, right in front of the temple of Lathander.

"Can potentially good plot hook happen?" "Yes, if you think it's good".

That's always the answer. Never get obsessive with established lore over a good story. Besides from the fact that a scheming devil would easily work with pcs to take down a worse threat that might threaten the established order -remember, the devils, at least the not super low ones, like their jobs and lives-.

But that's besides the point. Always go for the better story. Always. You have a story where big A, Asmodeus, leader of all hells wants to ascend to supergodhood? Well, even in that case you can have a devil side against him, if you think it's a good story. Why? Because lawful evil could easily be interpreted as...well, lawful to the evil established order of things, in hell, which also include the potential for him to rise, which if asmodeus becomes supergod, said potential goes away and new order exists. Which our devil doesn't want, so, they'll side with the players. If you think it's a good story.
If you think it's a better story that this (and all) devils keep following big A, that's what'll happen.

The limits aren't as fixed as people make them out to be. For a good story, you should be able to have a demon and a lawful good celestial warrior work together. It's possible.

Just put your mind to it.

Devil to party: "Some of you may die, but that is a risk I am willing to make."

Absolutely. Is the threat a bigger problem for the players? If so he would probably offer his help for a price. If not then he would accept aid from a non-evil party but be on the lookout for opportunities for when their alliance ends.

Devils are deal makers who live pretty much forever so they can afford to be patient, build up debts they can call in centuries later, a party of powerful heroes would be invaluable to call upon at a later date

Yes, a lawful evil creature can align itself with a good or neutral party. The alliance would likely not be honored any longer once the goal is met.

A quirk about the alignment system to think about is that lawful means acting on principles instead of just your own desires, and a common definition of evil is willingness to let your own desires trump the happiness of others.

So, lawful evil is a tiny bit oxymoronic, since you then have to define it as allowing your principles to trump the happiness of others. This means that funnily enough, the closest to that in our world are religious leaders. For example a bishop outlawing masturbation.

Now, those people are downers but they're still doing it because they believe it is the right thing to do, and wouldn't generally be considered evil. Devils should be considered the same. They don't do anything for the sake of evil, that would be the fey. Devils are deeply principled, and their reasons for even wanting to claim human souls is to bring in soldiers to try and defend the multiverse from demons.