This is worldbuilding for my Beyond the Wall game, but since it was most of the way to compatible with D&D 5e anyway, I went ahead and made the tweaks. My players have gotten really into hanging out with the fae at the regular seasonal markets, and are drifting pretty close to just signing up. This was a way to systematize that process.

Also, check out Brandes’ recent post about fae contracts for some largely compatible ideas.

The Bane of Iron

Most fae are weak to iron, the punishment of the Sun for interlopers from the shadow planes. This is not an inherent weakness of Sidhe, except insofar as most are fae. It is said that sometimes mortals that become full Sidhe essentially become adopted by a shadow plane, effectively becoming fae.

Oaths of the Sidhe

Each rank of the court allows another oath to be taken, and each oath taken cements that rank in the court. Most serious Sidhe courtiers have taken all of the oaths: it is the requirement to be considered full Sidhe and to truly engage in Sidhe politics. However, those who have taken any oaths are part of the hierarchy, and have enforced respect over those that bow to Sidhe sovereignty, particularly in Sidhe lands.

Oaths broken tend to result in grievous wounds, commensurate with the wrong committed.

Oathkeeping (Wisdom)

The most common oath is for the keeping of oaths themselves. This is the oath responsible for the famous Sidhe inability to lie.

My words will be my oath. What I say is true. What I commit to, I will perform.

Drawback: You cannot tell an outright falsehood (but can mislead through technically true statements). Your promises always count as an Oath.

Benefit: Gain Advantage on Wisdom checks to determine if someone is deceiving you. Make a Charisma (Intimidation) check when calling a promise due to force even a non-Sidhe to keep an oath to you (or accept commensurate consequences), DC equal to the target’s own Charisma score.

Hospitality (Constitution)

Another common oath, this is the one that protects others from the Sidhe as a guest. It is the only reason Sidhe politics can continue at the most cutthroat times.

While I share bread and drink with my hosts, I am their guest and they are mine. I shall respect their homes, and expect the same. Should they withhold their threat from me, so I shall withhold mine from them, until the guesting is through.

Drawback: You may not directly harm or even work strongly against guests and hosts after accepting/granting hospitality (but can politic towards eventual harm after guesting is over).

Benefit: Gain resistance to all damage dealt by someone who is part of a pact of hospitality to you (and this waives your need to avoid harming the aggressor). Gain Advantage on Constitution saves and checks made when you have guest right (e.g., against toxins or other poor conditions). Roll Charisma (Intimidation) against the target’s Charisma score when you are a host to force violators or those who will not swear from your home.

Demesne (Strength)

This is the oath that allows the Sidhe to build nations and employ diplomats. It is usually sworn after Hospitality, for it is that oath writ to a grander scale. It is the reason Sidhe can be driven off of even mortal lands, due to the respect of authority.

I shall respect territory, as I expect my own to be respected. Should I remain in land where I am unwanted, then this shall be war.

Drawback: You must leave an area when ordered by a rightful authority unless on a mission of declared hostility (and, unfortunately, church bells usually count unless you are expressly welcome, due to the general hostility of the church and their authority over mortal lands).

Benefit: Gain Advantage on Strength checks or similar rolls to erect fortifications or bar portals in your own lands. You, your mount, and your immediate retinue move 50% faster when moving in your lands to intercept invaders. You may automatically sense the strength and potential flaws of fortifications and other defensive measures nearby.

Gifts (Intelligence)

All fae have picked up the gift-giving system from the Sidhe, mostly because an upper class with very specific views on exchange of property quickly creates a culture of it. This oath ultimately serves to formalize ownership and prevent corruption through bribes.

I shall accept nothing that I am not owed. I shall give nothing without an expectation of a return in kind. My value comes from my deeds, not from the whims of others.

Drawback: You cannot accept a gift of item or service without providing something of similar value (owing a favor if you cannot immediately reciprocate); if you have provided services without a formalized gifting/quest structure, you can accept a gift as a way to settle this debt.

Benefit: You gain Advantage on Intelligence checks to appraise the value of items or services, and automatically succeed when they are offered to you as gift or for trade, allowing you to flawlessly detect counterfeits or other items with inflated values. Gain double XP for conspicuous consumption*. Spend Inspiration to have fate help track items that were stolen from you or to see that fortune returns them to your hands.

* In my campaign, the PCs earn XP based on spending cash on goods and services that make their characters happy but have no significant rules effect.

Craft (Dexterity)

Sidhe also have a propensity for games, riddles, and art competitions, as a way to establish dominance without bloodshed. This oath speaks to the cleverness required for true nobility, and is often one of the last oaths sworn by those fae that do not trust in their own intelligence.

I am wit, poise, and guile given form. Should one seek to test me in the domains I have claimed talent, and be it no true hardship, I shall prove my skills or acknowledge my superior.

Drawback: You cannot refuse a challenge to a competition over one of your proficient Skills or Tools unless it is obviously, actively dangerous to you (e.g., a distraction from a fight) or you admit that the challenger is better (you cede the advantage for winning to them).

Benefit: If you win a competition where you were challenged, you are owed by the loser similar to them promising you a favor (with the strength of the favor based on how much effort was required for the competition; a quick riddle game is not the same as a challenge to see who can topple an empire). You owe this in turn if you cede a competition to the challenger without competing, but not if you simply lose after giving it your try (in this case, the favor is minor because the stakes were small). You may learn to work dross and other ephemeral qualities into your crafts (essentially, crafting magic items).

Identity (Charisma)

Not all fae are vulnerable to use of their true names, but this is the oath that ensures it. It is often the final oath, as it is the ultimate claim of identity that allows full nobility.

My name is my own, though I may keep it safe. By my sigil, my will. By my name, my pledge. By my existence, my guarantee. There is none other like me.

Drawback: Statement of your True Name by an antagonist weakens* any of your mystical protections, as well as your mystical attacks against the target (and “statement” may be broad enough to include working your name into bindings or other magics). It also unmasks you of any magical or physical disguise.

* GM’s discretion, usually advantage/disadvantage or the equivalent for effects that don’t involve a roll.

Benefit: Attempts to impersonate you, even with strong magic, automatically fail against anyone that has met you. You automatically succeed on saves to resist being transformed without your consent, unless the aggressor has some kind of authority or broken oath to hold against you. Similarly, you stand strong athwart time and reality, and can ignore changes to the flow of time, causality, and local reality if you so desire (e.g., immune to magic like Slow, Time Stop, and other plot-related weirdness).