the geasa thing I just don't understand
A geas (geasa is the plural) is basically a taboo. However, as long as you keep it, you will be stronger in some way. If you break it, however, you will be weakened (and probably get killed, it's a good idea to keep them secret or your enemies will use them against you).
For exemple, an Emoulin fisherman could accept a Geas from a water fae that states "you shall eat no other meat but that of a fish". As long as he keeps to it, he will sail and fish better and be able to find his way back even in treacherous waters. Since the geas is basically "live from the sea", it makes sense that he'd be at ease with all things related to it.
Now, let's say that a rival, angry that he's getting so much fish, learns of his geas, he could slip some piece of bacon in his food, and the next time the fisherman goes to fish, well, at best he won't catch anything and at worse he won't ever come back.
This is why lies are so rare in Emoulin. With geasa flying around,
you keep your goddamn word (which is why poor Lirra doesn't get sarcasm and believes pretty much everything you say to her).
Given I doubt fishing will be of much interest for this quest, you can have "Protect the weak" (boosts your combat skill when doing so, but don't leave them to their fate no matter what) "Accept all challenges and show no mercy in them" (you pretty much can't lose a duel, but you better hope no drunken friend of yours challenges you as a jest or one of you will have to die) and so on.
Also, usually one needs to accept it, though in some cases, such as when one doesn't do what he is required to do, a geas can be forced (for exemple someone breaking the laws of hospitality could have one either to follow them and more, or to right the wrongs he caused others by breaking it in the first place).