A Geas placed a magical command on a creature to carry out some service or to refrain from some action or course of activity, as desired by the caster.
The creature had to be able to understand the caster, and while a geas could not compel a creature to kill itself or perform acts that would result in certain death, it could cause almost any other course of activity. The creature under a geas had to follow the given instructions until the geas was completed, no matter how long it took. 
It was known that a ronin, Rikako gained unique magic skills as a part of being put under a geas by a spirit of the Burning Sands. As a side effect of the geas, the fingernails on each of her hands had become long and black. 
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