The Infernalist (PrC).

[alhoon]

Infernalist:
Inffernalists are mages that draw their knowledge and Power from forces of the lower planes. They dabble in dangerous sorcery and consort with strange an evil powers.

Prerequisites:
Alignment: Any nongood
Feats: Improved Familiar
Skills: Arcana 5 ranks, Spellcraft 5 ranks, Planes 5 ranks
Spells: Able to cast 2nd lvl arcane spells. At least 3 spells with the evil descriptor or that need a powers check when cast
Special: The familiar must be an evil outsider with intelligence 8+ (Usually an imp or quasit).
Special: The character must Perform a special ritual, that the familiar shows to the character. In this ritual, the character contacts malign fiendish powers from the lower planes (or some aspect of the Dark Powers) to gain the powers of this class.
These powers may ask the character to perform some vile servive in return first.
The ritual itself is an act of ultimate darkness. If the character ever embarks in a quest to redeem for this act, he or she draws the attention of the Furies as described in DoDread. If the powers that give the character power demand another act as service, this quest doesn't force the character to make another power check.
As a result the character loses only one power check when gaining this class.

Class Abilities:
HD: d6
Class skills: Knowledge (Arcana, planes, religion), spellcraft, concentration, intimidate, bluff +1 that the DM chooses depending on the familiar and the nature of the forces the character consorts with.
Skill points: 2 + intelligence/level
Weapon & Armor proficency: none.
Good Saves: Will.

spellcasting: At each level of this class, the character advances in spellcasting ability as if he has gained a level in one arcane spellcasting class that he had before.
Also his familiar inproves likewise. The character doesn't get any other benefits.

Forbidden Arcana: The character cannot cast spells with the good descriptor.
If the character is a wizard, he can cast an extra spell/ spell level each day. This slot can only be used to memorize a spell with the Evil Descriptor or a spell that calls for a powers check. Each time the wizard gains a level, he gains a bonus free spell that he can add to his spellbook, on top of the two he gains normally. This spell can only be a spell with the evil descriptor or one that calls for a powers check.
If the character is a sorcerer, he gains one extra spell known/ spell level he can cast. This spell can only be a spell with the evil descriptor or one that calls for a powers check.

Evil incantations: Any spell the character casts with the evil descriptor or that calls for a powers check has a +1 DC to resist or dispel.

Fiendish Summonings: The character can use his link with the lower planes to summon or bind evil outsiders (mist or otherwise) at a +2 caster level.

Resilience: The character gains resistance to fire 5, damage reduction 2/good and a +1 natural armor bonus.

shield of the corrupted:The character is constantly protected by a protection from good spell. The spell can be dispelled (6th caster level) but the character can reactivate it as a free action in his next turn.
Paladins can detect the evil in this character with the use of their detect evil ability. Also Know alignment spells reveal that he is evil. Even if the character is neutral, the results of these spells and abilities show that the character is evil.

Taint of evil: If the character dies, the evil forces that gave him power refuse to let him return easily to the world. In addition to the perils and penalties of trying to bring a character back to life, the character must make a special will save to become raised or resurrected.
If he fails a will DC of 12 + Infernalist class levels + the power checks he has lost, the character cannot be raised.
If the character's fiendish familiar dies or the character dismisses it, the character loses the benefits of the Evil incantations, Fiendish summonings and Shield of the Corrupted and cannot gain another level in this prestige class until he finds another familiar from the lower planes.
Whenever the character advances a level in this prestige class, he doesn't immediately gain the benefits of the level. He must first perform a ritual to contact his fiendish masters and perform a service for them. The familiar instructs him about the ritual he has to perform. Once the service is completed successfully, the character contacts the lower powers again and gains the benefits.

Class level / Class abilities
1st / Forbidden Arcana, Taint of evil
2nd / Evil Incantations
3rd / Fiendish Summonings
4th / Resilience
5th / Shield of the Corrupted




NOTE: Probably there is at least one a similar prestige class somewhere in the books, perhaps more.

[The Nightmare Man]

Yes, I think there is.

From the Chaos Magic tome from Mongoose I believe...

[Jason of the Fraternity]

NOTE: Probably there is at least one a similar prestige class somewhere in the books, perhaps more.

I would also recommend reviewing the Demonologist (page 54) and the Diabolist (page 56) of the Book of Vile Darkness. While there are some significant differences between your and these prestige classes, Alhoon, I think that you might find some inspiration in them. The demonologist in particular has some similar abilities to the ones you propose for your infernalist.

[alhoon]

Thank you, however I don't have the BoVD and I don't plan to buy it. Also I'm sure there would be more PrC like this. I also don't have the Fiend Folio, but probably in a book about fiends, there would be about 2-3 PrC similar with this one in concept at least.

However, I just hope that someone (except me) will find some use of it.