"Shame fall upon you if you do not take me with you tonight"
Gráinne, casting her geis upon Diarmaid
The Geis (which in the plural must be said "geasa") is a fearsome magical enchantment, used not only by the druids, the casters of satires (magical enchantments sung by a member of the priestly class) or the poets, but by any inhabitant of Ireland and probably of the entire Celtic world. Kings were subject to a great number of geasa, as were heroes; only the druids seemed to be exempt. The geis involves two instances: > 1. A prohibition: it may consist of not eating the flesh of a certain animal, not going to a certain place, or not receiving any guest after sunset.
- The obligation: it may consist of a command directed at someone, with the threat of being killed or, more gravely still, dishonoured.
The potion that Tristan and Iseult supposedly drink by accident is the softened image of a geis, cast by Iseult to compel Tristan to love her. Cú Chulainn dies because his enemies have induced him to transgress one of his geasa, which ultimately leads to the transgression of all the others. The geis thus acquires a notable importance in social and religious life, underscoring the significant role of oral enchantments in druidic rituals.