They aren't called Forbidden for nothing. They are stronger than the usual maneuvers of the “parent” school, but at the cost of permanent penalties on the initiator. They always share the same favored weapons, skill, and count as their “parent” school for purposes of feats and other abilities that call for a specific school.
Forbidden Schools can only be performed out of one's pure free will for their own gain. Charmed/dominated/summoned/binded/cohorts/followers and similar cannot use a Forbidden School.
Forbidden Sign-Any one who dabbles into a Forbidden school ends up developing a physical quirk due to the heavy physical and mental stress it implies.
Forbidden Cost-Using a Forbidden school maneuver or entering a Forbidden stance makes you suffer a cumulative "Forbidden Price" equal to its level. So for example if you used a 3rd level forbidden maneuver and entered a 9th level forbidden stance, you would have a combined Forbidden Price of 12. The specific effects of Forbidden Price are different from school to school. Any penalties and damage self-inflicted by using a Forbidden School ignore any immunities, reduction, resistances, tought bottles, wishes, miracles, god intervetions, and similar.
Regular HP damage can be healed as normal, but only three ways are known for removing the penalties (like reduced max hp or lost skill points):
-The character repents and receives an Attonment spell. He must replace Forbidden School for another feat and any known Forbidden Maneuvers/stances for normal maneuvers/stances of the “parent” school. This is a one time deal. Should she fall on the forbidden path again, no attonment spell can save her now.
-The character kills, in direct combat, a martial iniator of the same “base” school that doesn't know any forbidden techniques but must be no lower than 4 levels from the forbidden iniator herself. A willing sacrifice or backstab on the night won't do, they must fight each other at the top of their capacities. Cohorts, summons, binded creatures and similar always fail to satisfy this criteria. However one is allowed to bring “friends” to help. Once the kill is performed, all the penalties suffered from the Forbidden Maneuvers of the respective school are removed.
-The character meditates to cleanse her body and soul, recovering from the penalties at the rate of 1 per hour. So for example Mystia uses Mysterious Song, losing 9 mental skill points. She could then meditate and recover 1 lost mental skill point per hour of rest.