A wizard who concentrates his effort in a single school of magic is called a specialist. There are specialists in each type of magic, although some are extremely rare. Not all specialists are well-suited to adventuring--the diviner's spells are limited and not generally useful in dangerous situations. On the other hand, player characters might want to consult an NPC diviner before starting an adventure.
Specialist wizards have advantages and disadvantages when compared to mages. Their chance to know spells of their school of magic is greatly increased, but the intensive study results in a smaller chance to know spells outside their school. The number of spells they can cast increases, but they lose the ability to cast spells of the school in opposition to their specialty (opposite it in the diagram). Their ability to research and create new spells within their specialty is increased, but the initial selection of spells in their school may be quite limited. All in all, players must consider the advantages and disadvantages carefully.
Not all wizards can become specialists. The player character must meet certain requirements to become a specialist. Most specialist wizards must be single-classed; multi-classed characters cannot become specialists, except for gnomes, who seem to have more of a natural bent for the school of illusion than characters of any other race. Dual-class humans can choose to become specialists. The dedication to the particular school of magic requires all the attention and concentration of the character. He does not have time for other class-related pursuits.
In addition, each school has different restrictions on race, ability scores, and schools of magic allowed. These restrictions are given on Table 22. Note that lesser divination is not available as a specialty. The spells of this group, vital to the functioning of a wizard, are available to all wizards.
Race lists those races that, either through a natural tendency or a quirk of fate, are allowed to specialize in that art. Note that the gnome, though unable to be a regular mage, can specialize in illusions.
Minimum Ability Score lists the ability minimums needed to study intensively in that school. All schools require at least the minimum Intelligence demanded of a mage and an additional prime requisite, as listed.
Opposition School(s) always includes the school directly opposite the character's school of study in the diagram. In addition, the schools to either side of this one may also be disallowed due to the nature of the character's school. For example, an invoker/evoker cannot learn enchantment/charm or conjuration/summoning spells and cannot use magical items that duplicate spells from these schools.
Being a specialist does have significant advantages to balance the trade-offs the character must make. These are listed here:
A specialist gains one additional spell per spell level, provided the additional spell is taken in the specialist's school. Thus, a 1st-level illusionist could have two spells--one being any spell he knows and the other limited to spells of the illusion school.
Because specialists have an enhanced understanding of spells within their school, they receive a +1 bonus when making saving throws against those spells when cast by other wizards. Likewise, other characters suffer a -1 penalty when making saving throws against a specialist casting spells within his school. Both of these modifiers can be in effect at the same time--for example, when an enchanter casts an enchantment spell at another enchanter, the modifiers cancel each other out.
Specialists receive a bonus of +15% when learning spells from their school and a penalty of -15% when learning spells from other schools. The bonus or penalty is applied to the percentile dice roll the player must make when the character tries to learn a new spell (see Table 4).
Whenever a specialist reaches a new spell level, he automatically gains one spell of his school to add to his spell books. This spell can be selected by the DM or he can allow the player to pick. No roll for learning the spell need be made. It is assumed that the character has discovered this new spell during the course of his research and study.
When a specialist wizard attempts to create a new spell (using the rules given in the DMG), the DM should count the new spell as one level less (for determining the difficulty) if the spell falls within the school of the specialist. An enchanter attempting to create a new enchantment spell would have an easier time of it than an illusionist attempting to do the same.