A friend will be running a Black Crusade campaign soon. A character idea I've had, is a fallen Inquisitor. I'm pretty big into the 40k lore, but I can't think of a good reason for them to turn their back on the Emperor, other than "Because they fell to Chaos". I'm wracking my brain trying to think of something he could have found out, or learned that would change his opinion. Most Inquisitors tend to be pretty well read though, and I can't imagine there's much that would shake their faith.

Secondly, what character class in Black Crusade do you think would be represent this? On one hand, I think a Psyker would, because a lot of Inquisitors are also Psykers as well, and I would just have to work from the ground up for the right equipment. The other option is a Renegade, which would give him some pretty good combat potential.

Well if the Inquisitor has genuinely fallen to Chaos, then he could be a Xanthite who got over-eager in his research and slowly spiralled down, constantly justifying his actions as "necessary for the Imperium's survival" and then after sliding so far, someone already of Chaos might have had a chat with him and essentially gone "stop fooling yourself". OR an Inquisitor might have despaired (it happens) after purging his 10,000th cult, feeling that there is only rot and death at the core of the Imperium, and so chose to switch sides. OR, he could have gone into deep-cover thinking he would figure out what makes Chaos tick, but then Tzeentch went "Just as Planned" and corrupted him so badly there was no going back. He could also have been a Phaenonite, which was a faction declared Excommunicatus Traitorus for playing with warp-stuff uninhibited, and yes they still clandestinely recruit. He could also have been successfuly Denounced, making him Excommunicatus Traitorus, and then he fled into the Screaming Vortex (or wherever your game takes place) because he felt he was "above those whiny losers at the Tricorn Palace". But if he's at the point where he is a Black Crusade PC, he has no illusions he is now a terrible person (if he thought he was cold before, he ain't seen nothin yet).

As for a specialty. If he was a more social type (read: CONFESS! CONFESS!!!!!!!!!! *turns thumbscrews tighter*) an Apostate might be your thing, as the Ruinous Ones would doubtless hone his speechifying ability to a mono-edge. If he was all about purging da heretics and the type to get his hands dirty, a Renegade might be better, to reflect his ability to handle himself in his militant pursuits. If he fell because of his heretical beliefs regarding technology, he could be a Heretek, to reflect his new friends in the Dark Mechanicus going "Ohhh, fallen Inquisitor, he will be a magnificent tool" and outfitting him with their lovely bionics. Of course, he could be a Chaos Psyker, to reflect his dabbling with forbidden Chaos loveliness such as pacts or artifacts, or perhaps he was just a psyker who fell.

An Inquisitor can fall for any number of reasons, or be considered fallen.

Have you read any of the material in the Dark Heresy lines concerning the philosophies present in the Inquisition? I suggest you do so if you haven't. It'll give you something to chew on for character background.

You could have an Inquisitor have been part of the ordo xenos, but got a little too interested in Dark Eldar and just what they do to preserve their souls from Chaos. Maybe he's hunting normal Eldar as well to see the properties of their souls?

Still considered heavily corrupted, but not for chaotic reasons.

Its not hard to see an inquisitor learning something about chaos that he realizes is the the "truth" and that the imperial creed was a lie all along to prevent others from reaching this greatness.

Alpha legion. They have a little chat with the inquisitor about where their loyalty truly lies, and afterwards the inquisitor decides they have a point. (don't want to spoil anything incase you haven't read the HH novel "Legion")

Maybe the inquisitor had no choice in using xeno/chaos tech to defeat a dire threat, but when he deafeted it another inquisitor saw it, blabbed to his colleuges how your Inquisitor has become corrupted and he had to flee into the warp/lawless parts of the imperium. He hasn't quite fallen yet, but...

Your inquisitor has infiltrated a chaos warband, atleast that was the plan, over time he has begun seeing things from their point of view more and more...

Your inquisitor is looking for a chaos/xenos artefact of such diabolic power, that it could wipe out whole fleets of the emperor's enemies. so what if 8888 psykers have to be sacrificed to khorne for it to work?

Your inquisitor reads a heretical tome in wich is decribed how the emperor allowed the heresy to happen so he could ascend to godhood. His mind snaps and he goes out to find the author: Lorgar Aurelius.

Apostate could also work. Or maybe even a pirate prince if your inquisitor took his own (requisitioned) craft with him when he fell.

_{Lots of great ideas in this thread! This is the idea that I've started thinking of after reading some of the ideas here:}

_{He started out as a Puritan Inquisitor. Very by the book, and "For the Emperor!". Slowly he started to realize that in order to truly fight Chaos, he had to start playing more on their level. Chaos was undoubtedly winning, and to him that reason was because they didn't try to control their emotions. They embraced all of their feelings that feed their Gods, and so started to give into these. Taking thrill in his hunt and kills, taking delight in his ability to plan ahead and outsmart those he was after, taking pleasure in the tortures he needed to inflict to get information, and secretly his despair at where he was heading. He gave into all of these emotions to better fight his enemy. Slowly he started to realize how the Imperium keeps everyone shackled and blind to the truth of the world. Denying people the right to be alive, and human. Another Inquisitor finally caught onto his treason, and while he was on a mission hunting a Chaos Sorcerer the Inquisitor ordered the entire city he was in leveled. The Sorcerer offered the Inquisitor a way out of the city before it was demolished, and he followed.}

One thing to read is the Eisenhorn trilogy. I'll avoid giving too much away but it explains that there are two schools of thought:

Puritans - Anything Chaos/Xenos must be destroyed.

Radicals - Use the weapons of the enemy against them.

Now the radical school is not usually in the best of lights and it is easy for an Inquisitor to slip from puritan to radical. Usually this can be simply due to time, their exposure to various events twisting their perception to the point that they, rather than burn that heretical tome, consider picking it up to use its forbidden lore to banish the daemon and in doing so cast doubt on themselves.

Inquisitors that are too radical may run the risk of being denounced by the others and finally being cast out but still their dark interests have left their mark. This is an opportunity that many above have alluded to.

In the book he is a puritan inquisitor but there are encounters with radicals and it explains the interaction between them, icy at best.

In terms of class therefore, there isn't really a suitable class. Not all inquisitors fit a specific template. Some spend their time charging into the front lines and slaying their enemy head on (adapting renegade archetype). Others might be the learning studious type who, when the need arises, use their subordinates or augmented tech to do the killing part (sage like characters) and then you also have some who are psykers (would need to be bound of course). There is no ideal archetype to use as a template since it depends on the background.