(Stolen shamelessly from an RPOL Game I'm in run by Onar Idein. All creative rights are his.)
In this cruel game you play solo as a Corruptor, an evil spirit slowly taking control of an NPC as you ruin her character, reputation, and life, step by step, by intervening in her behavior.
Your goal is to reduce her to an evil, humiliated, hateful creature with no morals and who is separated from everyone who loves her.
You have come from some dark immaterial place to feast on the ruination of your victim. She is a very beautiful, mostly innocent, mostly ordinary person with bright-souled good intentions. About you, we know very little, except that you are intelligent, conniving, invisible, and sadistic. You have the power to dictate gradually little choices that your victim makes, the actions she takes, and the feelings she feels. At first you can direct some of the things she says to people and her nonverbal communication. As your successes increase, and you manipulate her into doing more and more despicable things, you gain increasing power over other spheres of her activity, and you can dictate her actions more and more, until, in the end, you rule her life and she is entirely your character, no longer an NPC.
What you don't control is your victim's thoughts. This is the first danger you face. If you work too quickly, and by your manipulations lead her to do too many uncharacteristic things, she may seek help. That could be from a counselor, a dear friend, a spiritual advisor, and, worst of all, an exorcist or other person with the power to defeat you and to banish you to the darkness from whence you came. You must work carefully so that her personality changes gradually, so that you manipulate her to do things that she would not feel are too much out of her own current character. You do not want her to suspect that she is losing herself to you. Your back door is through her weaknesses and faults, the spots of selfishness that are more likely to give in. It is up to you to discover them in order to lay waste to her soul.
Another danger is the possibility of encountering other spirits, good and evil, who are working through other individuals. Your spiritual perception can detect them when they manipulate the actions of those around your target.
All this is set against the backdrop of your victim's life, which has its own unfolding, soap-opera-life drama.
HOW THE GAME WORKS:
You are assigned Spheres of Influence over your victim at the beginning. These include Actions that you can prompt your victim to perform impulsively--meaning without her thinking about them, because you made her do them--and Heart Strings, feelings that you can inspire in her that may lead her to make choices of her own that she may not have chosen. The power of each of these Spheres of Influence is rated in the duration that you can keep them working. The stronger the power rating, the longer you can sustain influence (and the more involved your posts can be).
The game starts with a description, not of your evil spirit, but of your Victim. You do not make a character sheet. If you have experience as a GM, that is probably a plus.
After every post I make, you intervene in the story with your post, continuing the narration that I started. You actually get to control the NPC in your post, and decide what she does--exactly as if she was your own character--BUT you can only do things that fit what she was already doing, or that reflect the Spheres of Power you currently control. My next post will continue from there. Everything that you declare should be consistent with the character as you know her, except for your corrupting influence.
If you overreach your power, and make her do things that are BOTH uncharacteristic of her established personality AND outside your sphere of power, you will attract a Good Spirit who will make her do things to improve her life and character, and that will thwart your attempts to ruin her life.
The one thing you can't narrate in your Victim's life is her thoughts, which will normally appear in my posts in italics. You only control her actions, which she will be thinking about and reflecting on in my posts. *Of course, as a spirit you can easily read her thoughts.)
Your interventions in her life will over time corrupt her personality so that, if you play your hand well, she will question less and the things she is impulsively doing at your will. Your control will grow, and you will gain new Spheres of Influence and increase the power and duration of the ones you already have. Eventually you may control almost everything she does and you can have free reign to corrupt her completely.
In short, the game will work a little bit like "round robin" storytelling, but you control only the actions of your Victim.
Now accepting prospective players as well as allowing someone the option to run this game. I've found it has one of the best formats for Play By Post and really creates an interesting tension for all parties involved. PM for more details/questions.