(1-5 point Flaw)
There is something you must or must not do, and your life, your luck, your magic and perhaps your very soul depends on it. It may be something that has always been upon you, a geas prophesied by Druids at your birth, a sacred oath or vow you swore, or a promise or bargain you made. Someone (with a capital S) witnessed it and is going to hold you to it. If you disobey, the consequences will be dire, if not deadly.
Characters may have several magical prohibitions or imperatives, and these may come into conflict. In Celtic myth, Cuchulainn had the geas to "Never refuse hospitality" and "Never eat dog meat." Three hags once offered him roast dog for dinner, and Cuchulainn died soon after. Consequently, most changelings keep their magical prohibitions and imperatives secret, lest they be used against them by enemies.
Storytellers should examine each prohibition or imperative and assign a point value to it, as well as to the punishment for violating it. Easily avoided circumstances, such as "Never break bread with a red-haired man," are worth one point, while more common, or difficult, things, such as "Stop and pet every cat you see," are worth two points, and particularly drastic or dangerous circumstances, such as "Never back down from a fight," are worth three (or more) points. Consequences are worth points as well. Automatically botching the next major cantrip you do is worth one point, having bad luck for the rest of your life is worth two, losing all your friends and worldly possessions is worth three, dying is worth four, and being deserted by your faerie soul five. Characters and Storytellers may come up with variants of these.
Traditionally, there is very little that may be done about geas, which are simply facets of one's destiny, and curses are devilishly hard to lift (and the Flaw must be bought off if they are). Characters who accidentally violate them may attempt to atone for their crime, fixing whatever they did wrong. A witch who has vowed to never eat any red meat, and then suddenly finds beef in her soup, might be able to atone for the trespass by fasting and sending checks to PETA. However, if a changeling violates an oath willingly and with full knowledge — and survives — he becomes an oathbreaker, one of the foulest epithets among changelings. Oathbreakers are psychically marked. It is virtually impossible for them to find a tutor or any sort of aid.