From Compendium of Worldly Lore
The demonologist is the quintessential practitioner of the dark art, toiling for long hours under his academic labors to harness the powers of the infernal planes. The vast majority of demonologists start off as wizards and most still practice their regular arcane magic but the mainstay of their studies is firmly rooted within the realms of demons.
Most demonologists are solitary individuals, for there are few willing to grant them the freedom to practice the summoning rituals necessary to call forth the greatest of powers without comment. The very nature of the demonological art requires knowledge and components that any of the civilized races would find distasteful to the extreme. In addition, the penalties for any error the demonologist himself makes during the summoning of greater powers can cause a great deal of harm to any around him as demons, freed from their shackles and at liberty to terrorize the material world for a short period of time, slaughter all around them.
There are, however, certain parties willing to risk the presence of a demonologist in order to further their own plans, whatever they may be. Powerful guilds and organizations or adventuring parties with few scruples may be welcoming of a demonologist into their ranks, who is likely to appear to be nothing more than a regular wizard until he actually starts to practice the black art.
Either way, demonologists are often regarded as strange people who rarely see the world as others do. After having faced the fiends of the infernal planes in direct and costly battles of sheer unadulterated willpower, few can remain completely unscathed. It is a tragic tale that many demonologists go into the practice with the purest of hearts, only to have their morals and standards constantly chipped and battered away until they become, quite literally, completely different people.
Role: Spell Utility
Characteristics: Walking the fine line, these casters are masters of the arcane act of Demon Summoning
Source: Encyclopedia Arcane: Demonology
Game Rule Information
The True Demonologist has the following game statistics.
Abilities: Constitution is very important to remaining alive, but perhaps the most important stat is Charisma for controlling and summoning Demons
Alignment: Any
Hit Die: d4
Starting Gold: N/A
Class Skills
The True Demonologist's class skills are Alchemy (Int), Concentration (Con), Craft (Int), Decipher Script (Int, exclusive skill), Knowledge (all skills, taken individually) (Int), Listen (Wis), Profession (Wis), Search (Int), Spellcraft (Int), and Spot (Wis)..
Skill Points at 1st Level: (2 + Int modifier) × 4
Skill Points at Each Additional Level: 2 + Int modifier
Class Features
All of the following are class features of the True Demonologist.
Weapon and Armor Proficiency: A True Demonologist is proficient with whatever they were already proficient with prior to taking levels in this class.
Demonology: Demonologists are versed in the summoning and controlling of demons and so may use the Art of Demon Summoning rules.
Spells per Day: A demonologist continues training in arcane magic as well as his own field of research. Thus, when a new demonologist level is gained, the character also gains spells as if he had also gained a level in an arcane spellcasting class he belonged to before he added the prestige class. He does not, however, get any additional benefits a character of that class would have gained, only the new spells.
This essentially means that he adds the level of demonologist to the level of another arcane spellcasting class the character already has and then determines spells per day and caster level accordingly. If the caster had more than one arcane spellcasting class before he became a demonologist, he must decide to which class he adds each level of demonologist for the purposes of determining spells per day when he adds the new level. Demonologist levels are never added to a class that uses divine magic.
Siphon Demonic Energy Su: At 2nd level, a demonologist gains the ability to Siphon Demonic Energies, adding them to his own magical strength. To siphon energy, a demonologist must make a successful Control check for a demon he has just summoned. Instead of giving it instructions as normal, he may instead draw upon its raw energy. This may only be done whilst the demonologist is preparing his spells. The time taken for the summoning is added to the hour normally required to prepare spells.
The demonologist will gain extra spell slots for one day, based on the Challenge Rating of the demon. Each point of the Challenge Rating will allow an extra spell level. These spell levels may be split or combined for additional spells as the demonologist sees fit, but he may not use them to take spells he could not ordinarily cast. For example, an osyluth (CR 6) could be used to obtain an extra 6* level spell, a 2nd and 4th level spell, 6 1st level spells or any combination in between. However, the 6th level spell could only be gained if the demonologist could normally cast 6th level spells. The demon returns to the infernal planes as soon as the energy has been taken from it.
Extended Instruction Ex: At 3rd level, a demonologist grows strong enough to compel the demons he summons to follow longer and more complicated instructions. The limit for each instruction issued grows to twelve words.
Lesser Control Su: At 4th level, the demonologist has the strength of will to compel demons to serve him for longer periods of time. The maximum time a demon can stay in the material world is increased to two hours. Each successive Control check to retain the services of the demon for longer will now allow it to stay a further two hours in the material world instead of just one.
Multiple Summoning: At 5th level, the demonologist has grown so adept in his art that he may now summon more than one demon at a time. The rules for doing so are covered on the Art of Demon Summoning
Demonic Vulnerability: As the demonologist's knowledge increases, so too does his ability to maximize the effect his spells have on the creatures of the infernal planes he may meet in battle. At 6th level, the demonologist may add +1 to his casting level whenever targeting a creature from the lower planes with arcane magic spells.
Full Instruction Ex: By the time he reaches 8th level, the demonologist has become truly skilled at controlling demons and forcing them to do his bidding. Even the most complicated instructions can now be given and will be obeyed. The limit for each instruction issued grows to twenty words.
Greater Control Su: The demonologist's mastery of his art is almost complete as he reaches 10th level. He is now able to extend his control over demons in the material world for much greater lengths of time. The maximum time a demon can stay in the material world is increased to three hours. Each successive Control check to retain the services of the demon for longer will now allow it to stay a further three hours in the material world instead of just one.