From Ardrana

A geas spell places a magical command upon a creature (usually human or humanoid) to carry out some service, or to refrain from some action or course of activity, as desired by the spellcaster. The creature must be intelligent, conscious, under its own volition, and able to understand the caster. While a geas cannot compel a creature to kill itself or perform acts that are likely to result in certain death, it can cause almost any other course of action. The geased creature must follow the given instructions until the geas is completed. Failure to do so will cause the creature to grow sick and die within weeks. Deviation from or twisting of the instructions causes a corresponding loss of strength until the deviation ceases.

Note: This version of the geas originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.