Oppositional Schools (CWH)

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Each school lies in opposition to one to three of the existing schools of magic. The specialist of the new school will be unable to learn spells from the schools that lie in opposition to his school.  
The first step in determining oppositional schools is to decide which existing school most closely resembles the new school. In some cases, this is obvious; for instance, our new school of metamorphics is most like the school of alteration. For less obvious cases, you can eliminate most of the possibilities by recalling which type of magic your new school is derived from (see the Types of Magic section above). A new school will most closely resemble an existing school derived from the same type of magic (see Table 4); for instance, if your new school produces Creation effects, it most closely resembles either the school of illusion or invocation/evocation. Once the choices have been narrowed to two or three schools, it should be easy to pick the school most like your new school. If necessary, try comparing your new spells to the existing spells in the Player's Handbook .
When you've decided which existing school most closely resembles your new school, consult the opposition diagram (Diagram 1). The school that lies directly opposite this school is the oppositional school. The school of metamorphics most closely resembles the school of alteration. The school directly across from alteration on Diagram 1 is abjuration. Therefore, abjuration is the school that is in opposition to metamorphics.
One to three oppositional schools may be assigned to a new school. If the new school is relatively weak (the weak school of greater divination has only a single oppositional school), only one opposition school may be needed. If your new school is relatively powerful, it is appropriate to assign it as many as three oppositional schools.
Look again at Diagram 1. The schools on either side of the oppositional school are also options for oppositional schools. For instance, additional oppositional schools for illusion are invocation/evocation and abjuration.  If it's appropriate for your school to have three oppositional schools, choose these three.
Most likely, your new school will be neither extremely weak nor extremely powerful. In this case, consider giving it two oppositional schools. One should be the initial oppositional school, and the other can be either of the adjacent schools. To decide between the adjacent schools, choose randomly or choose the one that seems less compatible with your new school.
We must choose the oppositional schools for our new school of metamorphics. Since it is neither exceptionally weak or powerful, we will assign it two oppositional schools.
We decided previously that the school of metamorphics most closely resembles the school of alteration. According to Diagram 1, the oppositional school of alteration is abjuration; therefore, the oppositional school of metamorphics is also abjuration.
According to Diagram 1, the adjacent oppositional schools for abjuration—and likewise, for metamorphics—are conjuration/summoning and necromancy. We need to choose one of them for our other oppositional school. Either conjuration/summoning or necromancy would be appropriate, but since the school of alteration already has abjuration and necromancy for its oppositional schools, we will choose the other combination. Therefore, the opposition schools for metamorphics are abjuration and conjuration/summoning.
Admittedly, the choice of the second and third oppositional schools is somewhat arbitrary, but remember that we not only strive for balance, but also for schools with unique advantages and restrictions. The DM always has the option to invent a rationale for the existence of specific oppositional schools. For instance, the energies employed by conjuration/summoning and abjuration magic might induce agonizing headaches in a transfigurist, making it impossible for him to learn spells from those schools