A priest casting this spell forces the victim of the spell to follow one specific law. The priest may choose any law prevalent in the area in which the priest and the victim currently reside. Thus, if a city has no laws about murder, the priest cannot command the person not to kill.
The victim of the spell is forced to obey the letter of the law to the best of his ability.
Thus, if a victim were commanded not to commit murder, he would go to any length to avoid murdering someone.
Since the essence of this spell is tied to legal (and not moral) interpretation, characters may find loopholes that will allow them to work around the law in specific cases or to ignore the law in light of extenuating circumstances.
When casting the spell, the priest must speak the law to the recipient in such a way that he can hear it. The victim is allowed a saving throw vs. spell to avoid the effect. If the save is failed, the victim will never willingly violate the stated law as long as the spell is in effect.
Legal thoughts can be negated by dispel magic. The victim of this spell never perceives anything wrong with adhering to the law, and therefore never seeks to have the spell removed.