Geasa

A geasa is a vow or obligation placed upon a person (usually a hero, such as Cuchulainn) in Celtic mythology. Traditionally, the doom of the hero comes about due to his violation of such geasa, usually by accident.

There is a considerable similarity between geasa (which are a phenomenon of Irish Celtic myth) and the foretold deaths of heroes in Welsh myth. This is not surprising given the close origins of all the variants of Celtic mythology.