[RPG] Is it reasonable to punish a paladin for breaking an oath to protect a creature they did not realize was a major antagonist?

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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.

Best Answer

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.

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