Backstory

The events of my homebrew campaign take place after the fall of a powerful mage tyrant, assumed dead. About 6 sessions ago, my players recovered a living horse and dead human from a "manhunter spider", used by bullywugs to track down persons by scent. They have come to adore the horse, naming it Wheatgrass and protecting it vigilantly. My paladin (will turn level 3 next session) just asked if they can make their Oath of the Ancients over Wheatgrass, swearing to protect this noble creature.

I had to restrain my excitement in agreeing to my paladin's proposal, because of a key piece of information I've left out: the horse is, in fact, the mage tyrant in disguise. Depending on the narrative choices made by my players, they may soon discover that the spider was sent to hunt down the mage, though they still might suspect the dead human rather than the horse.

The magic used by the mage to transform into a horse form is protected by Arcanist's Magic Aura (30 day casting), and my players have by misfortune avoided plot hooks that might have provided clues to the horse's identity. To be fair, they have not been given a very good chance of discovering the horse's identity until now.

Question Context

So when the Paladin turns level 3, they swear their Oath to the Ancients. The PHB states the following on oath breaking:

If a paladin willfully violates his or her oath and shows no sign of repentance, the consequences can be more serious. At the DM’s discretion, an impenitent paladin might be forced to abandon this class and adopt another, or perhaps to take the Oathbreaker paladin option that appears in the Dungeon Master’s Guide.

Protecting Wheatgrass is not one of the official tenets of the Oath of the Ancients, but my player proposes to include protecting Wheatgrass into this oath voluntarily. Now I am considering whether or not to impose a penalty in the event that the paladin attacks the mage tyrant. For example, if the mage tyrant revealed his true form, and the paladin attacked, perhaps the paladin would lose Oath feature until completing a nature-themed quest.

My Question

Are a Paladin's class features contingent on upholding voluntary tennets their oath?

Would it be cruel or unreasonable of me to punish my paladin for unknowingly making an oath to protect a villain?

Considerations

As a GM, I know I can "do anything". I am giddy about the possible outcomes, but also worried that my excitement might mask the possibility of seriously upsetting a player.

Is there anything in the rules the explicitly forbids this? As a GM, I can overrule the rules, but this is more likely to upset players, who might want to have notice of such changes before choosing a class. I believe strongly that the rules of the game are part of the social contract between GM and player, in setting expectations and giving players notice of the decisions that they make.

In favor of going forward, I would just add that there is something fittingly fey about this conundrum, which might work with the Oath of the Ancients.

You certainly can cause the Paladin to fall if they've amended their Oath to include a very specific creature, and are then compelled to attack said creature once they realize the deception.

You really, really, really shouldn't though

For several reasons.

The Paladin is being deceived

The Paladin thinks they're swearing their Oath upon an innocent horse named Wheatgrass, not the literal big-bad of their current arc. If they then are forced to attack the horse once they realize its true nature, they're not really attacking "Wheatgrass", the innocent horse they swore to defend. They're attacking "the Mage Tyrant", an evil mage that pretended to be the horse they swore to defend. This gets into some vague, philosophical territory, but I don't think it's reasonable to treat the Paladin's Oath to protect a single horse as binding the moment they realize that they were tricked.

The Oath of Ancients is a Permissive Oath

The Oath of the Ancients is, by design, a relatively permissive archetype, ranging from more hippie-like characters ("I bestow the blessing of Nature upon you and wish you good fortune!") to stoic, ancient guardians of nature ("I cannot permit you to bring harm to this sacred glade!"), and many variants within that spectrum. Swearing an Oath to protect a specific horse, and then having the covenant of that Oath depend almost exclusively on that one promise is far more specific and particular than the Oath of the Ancients is designed to accommodate.

The Character needs to Willfully break their Oath

I'm particular on the word "Willfully" because it implies more than making a mistake or being put in dire circumstances. If the Paladin discovers that the creature they've sworn to protect is actually going to try to destroy the world, then they haven't really been given a choice at all. Like, ostensibly they could choose to aid the Mage instead, but given the Paladin's character as established by their desire to swear an Oath over a horse, that's not a real choice.

Having the Oath features be dependent on this decision won't be fun for the Player

Remember that first and foremost, the role of the Dungeon Master is to ensure that all the players are having a good time at the table. Discovering that the horse they swore to protect was actually an evil mage will probably be a surprising and exciting plot twist... Learning that their Paladin features will now be contingent on them choosing not to cause harm to said mage will be a lot less fun.

Conclusion

My advice is to simply say, when the reveal comes, that their Oath was made "In the spirit of" protecting an innocent creature, not in the legalese of protecting this very specific horse.

As a DM, this is probably a bad idea.

This is the paladin's player trying to reach out and roleplay better, in a small and awesome way. That sort of behavior should be rewarded and encouraged, as it leads to better play. There are players for whom this would be awesome - the sudden-but-inevitable betrayal and being torn between a paladin's oath they made innocently and in good faith on the one side versus the good of the world on the other. That's rare, though, and your paladin likely isn't one of them. (If you're quite certain that they are one of them, go for it, but if you were, you wouldn't be asking this question, so....)

It is far more likely to make the paladin player super-unhappy. That sucks, and you shouldn't just inflict that on them.

There's an easy dodge, though.

All you have to do is say "no." As @Xirema noted, the Oath of the Ancients is super-vague. Say that it isn't intended to be specific to that level, and that the oath system doesn't support that. Some oath modification is cool, though, and you should support that. Work with them to come up with a somewhat-more-generic version of the oath that doesn't point out this specific horse by name, but instead refers to a general (if perhaps rare) class of creature... and one that, coincidentally, doesn't happen to apply to polymorphed mage-tyrants. You still get that feeling of shock and betrayal... but it's towards the villain, rather than towards you personally, and it leaves the paladin in question more flexibility about which direction they take their character afterwards.

As a GM I would want to use this coincidence in my campaign, but would not want to punish the player for enthusiasm and good role-playing.

Fortunately, there is a trope of having spontaneous acts of nobility turning out to be the reason the good guy's win, even if in the short run it seems foolish. I would try to work that in (which, as is often the case, is much harder in an RPG without railroading than in most kinds of storytelling).

I might say that the paladin is bound by the oath, but later have that work strongly against the mage-that-is-a-horse-that-was-a-tyrant. Maybe there is a magic item that works only to protect those who you have sworn a powerful oath to protect (and only if they have agreed to your protection). When the mage later tries to turn himself into a lich, the paladin can protect him from that terrible fate.

This kind of thing is hard to pull off without railroading, and it needs to fit into your specific campaign, but you get the idea - make the oath real but turn it into something positive. Possibly you may end up having to tell the player about it out of game if they are too upset about it, but that can work well with some players too. Or it may not work with your table, but something to think about.

An oath obtained under false pretenses is not a valid oath. But . . .

Paladins live in a moral world, and an oath is not a mere word game. It is a trope of djinni and demons that they keep to their promises to the letter, to the detriment of all who deal with them. But it should be noted - this sort of literalness is a sort of evil, as it is a deliberate perversion of the victim's desires. A paladin's oath, however, is not something to be perverted by carelessness, ignorance, or fraud.

An oath is a sort of contract, and in the real world, no court in the free world would uphold a contract which is entered into on fraudulent terms. If the paladin were to sign a contract with another party to protect the horse, that contract would be unenforceable once the fraud was discovered. The Upper Planes are surely at least as righteous as an American civil court.

However, there is a point upon which the paladin might face some consequences. The gods do not much like paladins who make solemn oaths lightly. It is for good reason that making vain oaths is the very first of the Ten Commandments! It devalues the connection between a man and his god and of the very concept of loyalty. It is a sin, a type of disobedience to the will of his god. And what is true in the real world is surely doubly so in the DD world, where the effect and meaning of an oath is actually tangible. The paladin should be brought to repentance!

Ideally, this should come about as part of the broader story. The paladin made a foolish oath, the consequences of which should be severe. If you can figure out a way to make the path to adventuring success parallel the paladin's path to repentance, then you will have a fantastic story arc. If not, at the very least the paladin should see some consequences for his folly.

There are a few ways you could rule it.

1. The Oath of the Ancients overrules the voluntary oath.


2. If he breaks his voluntary oath, he will get punished.


3. If he keeps his voluntary oath, he will break the OotA and get punished.


4. If he keeps his voluntary oath and tries to keep the OotA, he will not get punished.

Everything depends on what the player will choose to do. It is my opinion that the voluntary oath should not be counted as a part of the Oath of the Ancients except that the original intention was to protect nature (and life) which seems to be a theme for those who swear that oath.

His voluntary oath may conflict with the Oath of the Ancients.

According to the PHB, those who swear by the OotA are committed to preserving life and light in the world. If this horse, Wheatgrass, turns out to be a force of evil, one who spreads darkness throughout the world, you could rule that the original oath of preserving life and light could overrule his oath to protect this horse which wasn't a horse at all.

If he breaks the voluntary oath, he gets punished.

This is reasonable. One should not make oaths lightly, and this is a good way to show that oaths mean business. However, the severity of the punishment should be in keeping with the severity that the oath was broken. For example, if the paladin does not protect the Mage Tyrant when he is in horse form, that is an egregious break. However, if the paladin fails to protect the Mage Tyrant when it is revealed that Wheatgrass was never a horse to begin with, the punishment should be less severe.

If he keeps the voluntary oath, he could break the OotA.

Maybe our Paladin friend will put his voluntary oath above the Oath of the Ancients and join cause with the Mage Tyrant. This would call for a switch to Oathbreaker or a class change.

If he keeps his voluntary oath and tries to keep the OotA, he does not get punished.

Player creativity could come into play here. Maybe the Paladin could try to save the Mage Tyrant and prevent him from destroying all that is good and right. Maybe he could try to turn the Evil Mage to the good side, or maybe he will work his butt off to protect the Mage and prevent him from doing any harm to the world by hauling him off to prison without giving the world a chance to sentence him to death. Whatever the result, if he chooses this route, you should reward him somehow.