You have some magical condition that you must fulfill in order to use your magic. This can either be a prohibition or a compulsion.
Compulsive geasa are things you must do to work your magic. You must generally be doing it as you cast your spells, and at least occasionally otherwise. This should not be a minor compulsion, such as smoking, but something major.
Prohibitive geasa have time periods; during the indicated period, you must not have performed the prohibited act. In general, less bothersome prohibitions require a longer period of abstinence. If not specified, you must not have broken the geasa for the entire game session.
A few examples:
You should be
- Blindfolded
- Bound
- Completely healthy and unhurt
- Drugged
- Having sex (in a very broad sense)
- In sunlight
- Skyclad (naked)
- Touching the ground with bare feet
- Wearing a specific, distinctive, and easily identifiable costume
You should not have
- Been in direct sunlight this session
- Eaten anything today
- Had sex for a year
- Touched a man/woman/animal/whatever this session
- Touched the ground in any way for an hour