Geis

In Irish mythology and folklore, a geis (/ˈɡɛʃ/; [ˈɟɛʃ]; plural geasa) is an idiosyncratic taboo, whether of obligation or prohibition, similar to being under a vow or spell. The equivalent Scottish Gaelic word, also used in English, is "geas" ([ˈkes], plural "geasan").

A geis can be compared with a curse or, paradoxically, a gift. If someone under a geis violates the associated taboo, the infractor will suffer dishonor or even death. On the other hand, the observing of one's geasa is believed to bring power. Often it is women who place geasa upon men. In some cases the woman turns out to be a goddess or other sovereignty figure.

The geis is often a key device in hero tales, such as that of Cúchulainn in Irish mythology. Traditionally, the doom of heroes comes about due to their violation of their geis, either by accident, or by having multiple geasa and then being placed in a position where they have no option but to violate one geis in order to maintain another. For instance, Cúchulainn has a geis to never eat dog meat, and he is also bound by a geis to eat any food offered to him by a woman. When a hag offers him dog meat, he has no way to emerge from the situation unscathed; this leads to his death.

A beneficial geis might involve a prophecy that a person would die in a particular way; the particulars of their death in the vision might be so bizarre that the person could then avoid their fate for many years.

...
Wikipedia