I've actually walked the path of the corrupted once before, playing a witch under the 2.5 D&D rules...  (yes, 2nd edition, not third.). Essentially a witch in that system became more and more corrupted the more they used their powers, (FYI, a witch in 2nd edition did get a major power boost in exchange for this.)<br/><br/>I started out as a Lawful Good, (not lawful stupid) character who called upon the forces of darkness in order to gain the power to correct a great wrong, and do good.  As the game progressed, this witch, (a female one, who I based after the "Mimi Masters" character from Ninja High School,), started out as prim and innocent as a nun who was raised in a convent (not counting having a batty grandmother who taught her how to make a pack with the forces of darkness), but as her corruption progressed, she became more and more violent, more chaotic, and quite a bit more worldly as she learned what she had to do to retain her powers.<br/><br/>Besides a gradual alignment shift (one step at a time towards Chaos or evil, depending on her choices) she began to show physical demonic physical traits.  She could resist the corruption, (and successfully if she was willing to lose the use of her powers for the day or more she needed to spend in the constant meditation needed to fight off an increase in her level of corruption.) but several times, at critical moments, she had to chose between keeping her increased powers to be able to save her friends, or letting them (probably) die in order to save her own soul.  In the end, she had been reduced from lawful good to chaotic good, and gained small horns, glowing red eye, hooved feet and bat style functional wings, and was only a couple short steps from going from a conduit for unholy power as a force for good, to becoming the demon she was starting to resemble.  At the last moment, just as our band of heroes, (who had been surprisingly supportive and trusting of their more and more demonic companion, probably unwisely so), were about to face the final foe they would needed to defeat in order to save their world, the big bad turned out to be a conduit for the same demonic force that had been powering my witch character (and again, it was a very significant power up...)<br/><br/>As the battle was joined, the villain used his powers to shove my witch over the edge toward the point of no return... And she was forced into a final choice... Leave the path if good to win the battle, or resist and risk losing her powers forever.<br/><br/>Her friends realized what was happening, and told her simply to trust them... They would survive, and save the world without her...  FYI, this was NOT what the GM expected from them... Especially as my witch would (only be dropping to Chaotic Neutral.). I knew, however, that crossing that line meant she would never have the willpower to resist falling again, so she chose to trust her friends, and permanently sacrificed her powers rather than risk falling forever into darkness.<br/><br/>When the smoke cleared, the world had been saved, if barely, and my (ridiculously powerful) witch character was suddenly a powerless, and rather unimposing human woman with no special gifts aside from a bit more arcane and demonic lore than average, and a knack for alchemy... (pretty much turned into a 3.5 expert, minus any weapon proficiencies...)<br/><br/>She retired peacefully to a small farm, where she faded into the obscurity of a mundane life... with only faint regrets for the loss of her immense power.<br/><br/>So yeah, I've gone the demonic corruption route before.  I do like to have some creative input in the process, especially if it's an inevitable outcome, though.