I apologize if this is in the wrong category, it might be more of a general lore question:

Are celestials and fiends bound to the good and evil alignments?  I can't find any examples of an evil celestial or good fiend, both of which sound weird for me, but does such a thing exist?  (I found a reference to a "corrupted" celestial in the text of one 5e module - no spoilers - but it didn't seem to have a stat block attached... unless I just missed it.)  I seem to recall reading some lore that a celestial who loses their "good" alignment is no longer a celestial, and similarly that a fiend who loses their "evil" alignment is no longer a fiend.  For example, Zariel is a fallen angel whose creature type is marked as "fiend," which seems to support this approach.  Obviously, encountering something like this would be a case so rare it would be dismissed as myth but... are the rule mechanics for such a thing actually spelled out anywhere?  Is there something "in between" celestial and fiend for unusual cases of this sort?  

For my own campaign (and I can elaborate on the details if anybody thinks it matters), the most appropriate rule seems to be to give this creature a combination "fiend/celestial" type, and any effects/rules/abilities that trigger on either of those types, would apply to this creature.  (For example, a paladin's Divine Sense can detect both fiends and celestials, meaning such a creature would register with that ability... "but you can't tell exactly which one it is, for some reason... and that's never supposed to happen, yet there it is.")  But does any kind of precedent exist in the rules for that kind of thing? 

Also, of course, I'd be interested to hear if anyone else has dealt with a situation like that, and how you handled the mechanics of it.

The way I play alignment is that it follows behavior, not the other way around. If an evil fiend starts acting good, it's a storytelling opportunity rather than a barrier to mechanics.

While a lot of the lore has been scrubbed, generally, the denizens of the outer planes exhibit alignment not just as the sum of their actions, but also as the expression of their being.  So a devil is lawful evil, and if it ceased to be so by its actions, it would also cease to be a devil (the opposite is what supposedly happened with Zariel, who was formerly a celestial).

So while I would argue yes, an outer planes being can change their nature/alignment (which would be rare indeed), it would come with a physical change of their form as well.

So, to be clear, are you looking to operate within an existing setting or homebrew? If it’s homebrew, then don’t get too wedded to the lore, it’s your setting to play with. Within an existing setting I’m not sure what kind of precedent there is for this. If nothing else, I’d say it would take something pretty world-shaking to shift a fiend to Good. The lore for Zariel suggests that it was exposure to the corruptive energies of the Lower Planes that helped cause her fall, and if you look at some other material there’s hints Asmodeus himself was nudging events along. Basically, if you’re trying to stick to official lore, the indication is it takes exposure to/interference by the opposing force to put a Fiend or Celestial in a position to make the change.

Regarding the creature type aspect, so far I don’t believe there have been any supertype hybrids/combos. A Dracolich is simply Undead, not Dragon/Undead, for example. Again, you’re the final arbiter, especially for homebrew, but the official design format seems to call for selecting a singular type.

It really depends on how you want to run it and how you want to implement alignment and good and evil in your game world. For PCs, I run it as Texas describes, actions determine alignment, a character will start with an alignment and if their actions indicate that the alignment doesn't fit the behavior of the character then the alignment will change.

However, NPCs are different since they are controlled by the DM. The DM can introduce good or evil NPCs of any race. In this case, though you are asking about fiends and celestials. In D&D lore, these are supernatural creatures from planes of existence that in part define their alignment.

There is no hard and fast rule I can find about how to treat atypical celestials and fiends in 5e.

The monster manual has some creature definitions and a section about "Angels" that might be useful. From the description, it sounds like you can have a fallen angel that may still be a celestial/something else/or a fiend. The fiend creature type description mentions that good fiends might be almost inconceivable but it would be up to the DM if they wanted to have something like that in a game - which really depends on how you want to run it.

Some folks consider an evil celestial or good fiend to be a contradiction in terms. If a spirit from the outer planes is good - it is a celestial, if it is evil - it is a fiend.

Other DMs (and the text on fallen angels) appears to be more nuanced. A fallen angel loses their connection to the deity that created them but they don't necessarily become evil. They made a mistake and the text allows for the possibility of that mistake being redeemed. Alternatively, the fallen celestial might choose a path of darkness and become a fiend by choosing evil. In the end, the creature type might still be determined by the creatures actions with good being associated with being a celestial spirit and evil being associated with a fiendish spirit, but one evil action or one good one might not be sufficient for a celestial to lose more than their connection to their diety or perhaps for a fiend to lose some connection to the plane where they were created. This is all up to the DM though and will vary depending on how they run their game world.

Can you have "gray area" celestials or fiends that may have taken a few actions that might be contradictory to their alignment - up to the DM. Some DMs would rule that it would be impossible for such creatures to consciously take an action that does not conform to their alignment - thus they need to be "tricked" into such actions that might cause them to lose their status. Other DMs would have these intelligent creatures decide whether good or evil actions best serve their immediate goals and have them behave accordingly.

Here is some text from the MM and DMG

"Celestials are creatures native to the Upper Planes. Many of them are the servants of deities, employed as messengers or agents in the mortal realm and throughout the planes. Celestials are good by nature, so the exceptional celestial who strays from a good alignment is a horrifying rarity. Celestials include angels, couatls, and pegasi."

"Fiends are creatures of wickedness that are native to the Lower Planes. A few are the servants of deities, but many more labor under the leadership of archdevils and demon princes. Evil priests and mages sometimes summon fiends to the material world to do their bidding. If an evil celestial is a rarity, a good fiend is almost inconceivable. Fiends include demons, devils, hell hounds, rakshasas, and yugoloths."

"Fallen Angels. An angel’s moral compass grants it a sense of infallibility that can sometimes spell its undoing. Angels are usually too wise to fall for a simple deception, but sometimes pride can lead one to commit an evil act. Whether intentional or accidental, such an act is a permanent stain that marks the angel as an outcast.

Fallen angels retain their power but lose their connection to the deities from which they were made. Most fallen angels take their banishment personally, rebelling against the powers they served by seeking rulership over a section of the Abyss or a place among other fallen in the hierarchy of the Nine Hells. Zariel, the ruler of the first layer of the Nine Hells, is such a creature. Rather than rebel, some fallen angels resign themselves to an isolated existence on the Material Plane, living in disguise as simple hermits. If they are redeemed, they can become powerful allies dedicated to justice and compassionate service."

DMG:

"The planes with an element of good in their nature are called the Upper Planes, while those with an element of evil are the Lower Planes. A plane’s alignment is its essence, and a character whose alignment doesn’t match the plane’s alignment experiences a sense of dissonance there."

Are celestials and fiends bound to the good and evil alignments?  

If you want them to be, yes. If not, then no.

does such a thing exist?

on Earth, in our current timeline, under the current circumstances, there are hundreds of stories about it, and redemption is a key factor, so Yes, they exist (for variable definitions of exist).

are the rule mechanics for such a thing actually spelled out anywhere?

Is there something "in between" celestial and fiend for unusual cases of this sort?  

But does any kind of precedent exist in the rules for that kind of thing? 

Hmm. Not that I can find beyond the fact that Alignment is a guideline and can change. However, I am not a living reference manual, and can only speak tot he core rules because I do not use nor know the official published settings lore beyond very superficial levels.

All of that said (and I did it that way to answer you specific questions), I have good devils, good demons, evil angels, vile , vain, and varied spirits of the outer planes all over the place -- those are th e stories and myths that people have historically used to help guide folks towards better life decisions. As in, in real life.

Game wise, the likely intent is or was to make those fixed for the majority of being in that realm -- but that is not a hard and fast rule (if for no other reason than nothing is a hard and fast rule in D&D) even if it was RAW and RAI.

I had a rather horrible event following a war among the gods, where they ripped the solar system out of the known universe (ours, for trivial reference) and set apart, creating the planes and their associated dimensions and populating the dimensions with different kinds of beings ( of which devils, demons, angels, valkyries, hags, wraiths, etc are all merely examples) who are, themselves, still subject to the same foibles we mere mortal beings are.

In mainstream d&D, Angels are Lawful good or whatever, and so stick around the lawful good plane -- but they could fall, and be barred from entry forever, unless they redeem themselves -- if they want to.  There is an entire collection of books by Debra Dunbar about exactly that, lol.

So, the easy part handled, as The_Ace_of_Rogues says above, the real question is "what setting -- what world -- is this around? is it your own, your homebrew?  Then go for it. Is it Forgotten Realms and you want it to at least kinda sorta stick to he bigger place?  Well, then you get to choose and decide if you want to do it, but there isn't a lot of rules aorund that yet -- though their may be, and perhaps at the next playtest you should add that as a suggestion in the survey.

In short, if it is a sentient and sapient being, it gets a choice, just like us. Most will tend towards one thing, but that does not mean all of them are that way -- and it doesn't mean that once they change they cannot switch back.

I mean, think about the hilarity that would ensue if you have a tiefling whose ancestor was a good demon?

Personally I actually tend a little away from the pure choice aspect for planar beings. They’re formed from the raw concepts of/associated with the appropriate alignment(s), so they don’t actually have the same sense of self mortals do. Plus honestly for me it makes everything a little too samey if the embodiments of cosmic forces are ultimately just people too.

Quote from The_Ace_of_Rogues >>

Personally I actually tend a little away from the pure choice aspect for planar beings. They’re formed from the raw concepts of/associated with the appropriate alignment(s), so they don’t actually have the same sense of self mortals do. Plus honestly for me it makes everything a little too samey if the embodiments of cosmic forces are ultimately just people too.

Well, same -- the point being we can do that, lol.  Since my Planes aren't alignment based I have a lot more flexibility, but Angels are still angels and wonder why anyone else would want to be anything else, Valkyries are still half drunk warrior women looking for a good fight and good time, Demons are spiritual creatures who live off emotion, devils are are into cruelty and getting the best flavor from their conquered meat, and so forth.

But I have seven planes in total, each with a Mortal Realm that is a reflection or the primary world (and one of which will be for the DDB version of the setting once I finish the lore book).   Lordy I gotta get off of here and finish it, lol.

From the monster manual:

The alignment specified in a monster’s stat block is the default. Feel free to depart from it and change a monster’s alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there’s nothing stopping you.

Thank you all for the input on the topic... and so quickly too!

To clarify my situation: I'm running a fully homebrew campaign in a homebrew setting.  I like to work within the general guidelines of existing lore, but I'd rather just tell a decent story and nudge the game mechanics in whatever direction they need to go for that. 

In this case, we have an NPC that is formerly a fiend- specifically, an incubus/succubus (shapeshifter, so its true form is neither male nor female... that's kind of beside the point) who escaped from the lower planes because the material plane is "just so much fun!"  Still chaotic evil, but after several adventures with the party this fiend ended up helping them rescue one of their friends from the lower planes, at great personal cost.  They were literally torn apart by the legions of the nine circles, as the rest of the group made their escape (in an act of defiance loosely inspired by Greed from Fullmetal Alchemist: "these people are mine, you can't have them").  Since then, this NPC has been "resurrected" (more or less, it's a long story) and it seems like the "evil" alignment no longer fits, so they've lost their fully demonic form in favor of something more resembling a fallen angel.  I'm not sure "fiend" is necessarily the correct type now, but I can't think of what else would qualify.  Not celestial, certainly.  Fey?  Monstrosity?  Nothing else seems quite right.

I landed on the "fiend + celestial" combination of types based on the idea that they've progressed beyond their infernal nature, but not fully.  So a spell like Hallow that triggers on creature type, would affect them:  a fiendish deity that is an enemy of the celestials, no longer wants this character; at the same time, they haven't fully shed their fiendish origins in the eyes of the celestial deities.  (They spend a lot of time waiting outside of temples whenever the party has to visit one.)  It just seemed like a very awkward way to apply game mechanics to this, and I feel like there should be a better option, but I can't come up with one.

But it sounds like I'm not contradicting any long-established lore by allowing the NPC to remain a fiend after they've clearly shifted to a neutral alignment, which is the main concern I had here.  

3e had the outsider category, which was not tied to alignment; I'm not sure why 5e decided to split them up. It might have to do with the choice to make alignment essentially undetectable (there are no spells that either detect or rely on alignment) combined with an idea of "but it really should be possible to admit in angels while not permitting in devils". If you go with the 3e version, your creature is now a neutral outsider.

You mention that this character waits outside temples.

Something interesting in my games: if you haven't been baptized to serve that deity, you cannot enter that temple Can't evne affect it. Gets really funny when people went to a Shrine the first time and tried to steal the artifacts and couldn't even touch them. 

This runs deeper, and gets more complicated in-game, but as a thing that happens, it has some interesting effects (if they aren't baptized, most buffs won't work on them, etc).

Perhaps you could set something up that baptizes the ex-fiend/fallen celestial....

... let the sparks fly (literally).

Quote from AEDorsay >>

Perhaps you could set something up that baptizes the ex-fiend/fallen celestial....

... let the sparks fly (literally).

It's funny you should suggest that.  My players just decided tonight that they wanted to find out whether holy water would still harm the not-quite-fiend.  It turned out that rather than hurting, any holy water that touches them will rapidly evaporate.  Which prompted one of the players to start asking, "so how do we redeem them the rest of the way?"  A literal baptism might be difficult to accomplish, but some other equivalent ceremony might have interesting effects.  If celestial deities start to accept this former demon, and it starts to challenge everyone's perceptions of what fiends and celestials are, well... some powerful beings on both sides may end up deciding that Something Needs To Be Done about this troublemaker.  Could be fun.  If the players decide to pursue this, I might turn that into a continuing story arc.  Sometimes the best plots are the ones your players pick for themselves, right?

Quote from Thoughtwire >>

Quote from AEDorsay >>

Perhaps you could set something up that baptizes the ex-fiend/fallen celestial....

... let the sparks fly (literally).

It's funny you should suggest that.  My players just decided tonight that they wanted to find out whether holy water would still harm the not-quite-fiend.  It turned out that rather than hurting, any holy water that touches them will rapidly evaporate.  Which prompted one of the players to start asking, "so how do we redeem them the rest of the way?"  A literal baptism might be difficult to accomplish, but some other equivalent ceremony might have interesting effects.  If celestial deities start to accept this former demon, and it starts to challenge everyone's perceptions of what fiends and celestials are, well... some powerful beings on both sides may end up deciding that Something Needs To Be Done about this troublemaker.  Could be fun.  If the players decide to pursue this, I might turn that into a continuing story arc.  Sometimes the best plots are the ones your players pick for themselves, right?

 yay!

So, a note about baptism: Baptism is a ceremony in many different religions, and only a few of them use water.  Some use fire, some use sand, some use charcoal, some use a kind of ordeal (like a vision quest), others have a whole ritualized thing where the supplicant is breathed on.

Mayhap there is a rarely used baptism?  The symbolism and ritual acts of the ceremony always carry some of the weight.

Disclaimer: I didn’t take the time to read all the posts here.

In Descent Into Avernus, there’s a little part about a brain-damaged bearded devil whose alignment is chaotic good, named Krikendolt. Notably, casting heal on him restores his alignment to lawful evil.

Make of this what you will.