Illusionists form a sub-class of magic-users, and in most respects they conform to the characteristics of the latter.
To be an illusionist, a character must have a minimum Intelligence of 15 and a minimum Dexterity of 16 (a high manual dexterity is required in casting of the spells used by this class).
An important difference between illusionists and magic-users is that the former do not gain any experience bonus for having high scores in their required abilities of Intelligence and Dexterity.
While being equal, or even slightly inferior, to normal magic-users in most respects, illusionists have different and highly effective spells to employ. A perusal of the number and types of spells usable by illusionists will reveal that they are at least as powerful as normal magic-users and possibly slightly more potent at very high levels.
The power of illusionists due to their spells is offset, however, by the limitations placed upon the magic items they can use.
See The Magic-user. Illusionists do not need the spell read magic or anything like it in pursuit of their profession. All illusionist spell books and scrolls are written in a secret tongue which every apprentice learns from his or her mentor. This arcane and difficult language is common to all phantasmal magics, and is necessary for illusionistic conjuring.
Use of Magic Items. The magical items usable by illusionist class characters are enumerated below:
Fabrication of Magic Items By Illusionists. Illusionists are capable of manufacturing those magical items which create or sustain illusion. They may do so at 10th level (Illusionist) and above; this is done in much the same manner in which regular magic-users create magic items.
Though different spells are employed, the process of fabrication of magic items which illusionists use is not really very different from that used by magic-users. It is almost exactly similar as regards costs in both time and money.
Scribing Scrolls. Some processes are also nearly identical, such as the making of scrolls, which may be done at the 7th level and up.
Fabricating Non-Permanent Magic Items. At the 10th level, illusionists may be able to create one-shot or charged magic items, things without a permanent dweomer, such as potions or a wand of illusion, for example. Such items are really merely storehouses of magical energy which can be released in various ways.
Fabricating Permanent Magic Items. Beginning at 14th level an illusionist may attempt to make items with a truly permanent dweomer, such as a +1 dagger or a ring of protection, for example.
The basic thing to remember if details are in question is that illusionists are a sub-class of magic-users, and except for what has been outlined above, what applies to magic-users applies to illusionists as well.
It is possible for an illusionist of 11th or higher level to construct a stronghold and clear the countryside in a 10 or 20 mile radius of all monsters, thus ruling an area much as a noble. If this is accomplished, a revenue of 5 silver pieces per inhabitant per month is generated in the territory ruled.
Illusionists strongholds will often be covered by illusion to appear as a ruined place, mound of rocks, or huge castle.