I’ve played a couple of specialized Magic-Users while playing AD&D and 3rd edition D&D. I liked the concept in a way, but at the same time it always felt a bit like min-maxing; the benefits of a specialist in the regular D&D rules were always far greater than the drawbacks.
What I find strange is that this should be some sort of sub-class. Specialized magic-users are defined by one thing; the kind of magic they use. This option, however, is completely open to a “normal” magic-user. Want to be an illusionist? Learn a lot of illusion spells and use them as your preferred weapon of choice. To my mind, that makes you an illusionist.
I do like the general idea that a magic-user would choose to specialize in a type of spells, though. A mixture of circumstances, personality and availability should probably influence the spell repertoire of all magic-users, and some would naturally choose a more narrow path. They research new spells within their chosen field and attract apprentices with the same focus, and gradually an entire school, cult or college might be founded.
I use specialized mages in my game, but I do it in this way; they have access to unique spells (as in distinct from those in the regular rulebook) but that access is not restricted by rules but by circumstance. Sometimes a special pact or action is required to harness their type of magic. Sometimes, casting the spells themselves leaves a mark on the caster which will over time set them apart from others. Most times, it is simply a matter of finding someone to teach you the specific magic, which might require membership in a specific order or living by some kind of code.
- The dreaded Necromancer is nothing more (or less) than a mage who seeks knowledge of the undead and learns spells which let him create and control them, but among these twisted souls knowledge of some powerful and forbidden rituals are passed.
- The Viper Mages of Al’Kulia fuel their unique spells with the venom they must constantly saturate their blood with, and the marks this leaves on their body strikes fear into all inhabitants of the Khalifate of Imrah.
- The Fire Walkers live ascetic lives and strive for bodily perfection in order to master the difficult somatic components of their unique brand of spells which harness fire and heat.
- The Order of the Seven Secrets is a society of mages who share a few unique spells used for scrying, but which most importantly teaches a special ritual which opens a portal to a sealed fortress in the Astral to which only order members have access.
I’ve prepared one special example where the drawbacks are very much tangible – the heretics who learn the spells from the Liber Heresiac, either to protect themselves from the prosecutions of the Trinity Church or because they resent the church for some other reason. Casting these spells will mark you as a target for the church, but the spells themselves are potent weapons to use against those of the faith.