Korihor is a character who turns up in the Book of Mormon in Alma 30.
Korihor is an atheist. He works hard to try to tell people that they don't have very good reasons to believe what they believe, and maybe they'd better take a good look at their leaders and their beliefs. He tells them that Christ will not come, because why would you believe someone who says he knows the future?
Korihor ends up having a showdown with Alma. Korihor welcomes to opportunity for dialogue, and explains why he is trying to get people to doubt the gospel. Eventually, Korihor keeps saying that he won't believe in god without proof, so Alma gives him proof... he strikes him dumb by god's power!
Here is where the nefarious aspect of Korihor's story comes out. Alma accuses Korihor of lying about Alma and other church leaders' taking advantage of the people, while in actuality they don't get a dime for their service. The account seems to support this accusation. Alma also says "can you prove that there isn't a god?" and Korihor is duped. Finally, the truth comes out: Korihor had been approached by Satan (in the deceptive guise of an angel) and set on his blasphemous task. He is the literal, willing servant of Satan, who manages to preach so subtly that even he starts believing his own lies.
Korihor is a very cleverly-made caricature of a skeptic. Initially, his arguments seems reasonable enough, and he seems to be concerned for the welfare of his fellow citizens. However, the righteous servants of god soon reveal that he is a fake who is literally in the employ of Satan himself.
The beauty of the Korihor story is that it gives LDS theology a delightful straw-man with which to equate all atheists, skeptics, and critics. Sure, these people might seem like they are honestly out to make a point, but they are all really just working for Satan for their own selfish purposes.
Now, I can't honestly speak for all atheists, critics, and skeptics. But I consider myself a skeptic, atheist, and critic of religion, and I haven't seen any angels. I know some accuse the leaders of the church of taking the money of the church and enjoying their positions of power, but I honestly think that they probably work pretty hard for what they believe is right and correct, and don't get all that wealthy doing it. I don't find a whole lot of similarity between Korihor and myself. I'm content to leave those who are not interested in skeptical inquiry alone. I haven't talked to Satan lately.
But I do happen to agree with Korihor on a few points. I don't feel that there is sufficient evidence generally available to justify the insistence on the existence of some intelligent, paternal god. I fear that religious leaders intentionally obfuscate the issues with their rhetoric, though I think that in many cases their intentions are good enough.
The point of Korihor is to show the LDS church that all those who criticize their unquestioning belief must be evil satanists with ulterior motives. I must be just like Korihor, right? The thing that makes Korihor really effective is his humanity, his believability as he is introduced in the text. Subtly and slowly, it is revealed what a real liar he is. If he had been presented immediately as a satanic man with clear ambitions of power and influence, he would be hard to compare to a lot of skeptics out there, but as he starts off as a more believable character, it is easier to equate actual skeptics with him and then assume that they are hiding similarly-nasty secrets.
All-in-all, the story of Korihor is a master stroke in creating preconceived prejudice against those of a more skeptical nature.