Essential Material: Geis (Patreon)

The curse of the ancient Celts. A form of spell wielded by the peoples of Ireland and Scotland, commonly seen in the mythology of the Ulster and Fenian Cycles. Has equivalents in other cultures, as well.

When one is broken, great misfortune befalls the oath-breaker. 

The primary and most commonly known form of geis is a form of contract or oath. Cúchulainn made several of these, including the oath to never eat the meat of a dog, and the breaking of this geis is what led to his eventual death. In such cases, there is often a tacit or explicit agreement by the person or people undertaking the oath that they will abide it. This form also applies to geis that are placed on positions of power, in that a person taking such a position agrees by the act of taking it to the Geasa that are placed upon it.

However, there is also a secondary form that exists as a curse one places upon another, regardless of consent. This form, utilized by Grainne against Diarmuid of the Love Spot, is one that requires power and authority over the one being bound, such as a princess or queen over a knight or a goddess over a mortal man.

Of note, the form of Geas Rin Tohsaka uses in the Heaven’s Feel route of Fate/stay night is the Germanic equivalent, a weaker binding curse that cannot dictate action, only forbid it. As a form of binding, it is absolute, but as a curse, it is a weaker effect.