Main Plot
Nathaneil is abandoned by his parents so he could study as a Magician. He becomes apprenticed to Mr. Underwood, but largely self teaches himself at his masters private library. Disastrous and complicated events soon follow when he summons, and attempts to control, a five thousand year old djinni named Bartimeaous.
Summoning
For ordinary summons, a magician must draw a pentagram around himself and at the spot the demon will be summoned at. Although the largest summons can require four magicians, only a single spellcaster is required to summon even afrits. Incense, candles, and inscriptions are also required. Aside from that, only speaking the demons name and a short incantation is required.
Their are other methods, of course, such as using "summoning horns". This method leaves the user unprotected against the demon, however, and the only remains of the first person to blow such a horn is, as stated by Bartimeaous "blackened fingerprints"
Deamons
There are six classical levels of deamons. [citation needed]. These include Marids, Afrits, Djinni, and Imps in descending order. There are other classes, but they are generally either too powerful or too weak to be of any use. They are invariably deformed creatures, sporting odd numbers of limbs and sensory organs. They usually hide their true forms under disguises. The number and quality of disguises of a demon is usually a good indicator of the demons intelligence.
Controlling demons is extremely difficult. While inside their summoning pentagram, they must listen to and promise to obey any orders their masters give. In addition they are most vulnerable to a variety of painful spells while in this state. Once outside the pentagram a magician can only hope the demon found no loopholes in their commands.
There are a variety of spells available to harness demons. Most common are the methods that allow one to bind a demon into an item or object, insuring it will not run away or otherwise cause mischief without your knoledge. Also available are very painful spells. The trump card of a magician, when all hope is lost, is a complex spell that binds a demon inside an object for 100 years. Some magicians purposeful delay the effect of the spell forcing the demon to cooperate in hopes that the magician will cast the counterspell in time.
Planes
Levels of magical state, of which their are seven. They are not treated as different places, but as different ways of viewing the same ___location. For instance, Bartimeaous can disguise himself so as to look invisible on about five planes. Powerful demons capable of seeing on the sixth or seventh planes, however, would see through his disguise immediately. Even while invisible, a demon can still be affected by, and affect, physical objects. As a general rule, the more planes you can see the more powerful you are.