Power corrupts. We know that. And there are always those that will feed from those with power. We know that, too. What is always good to remember, though, is how sweet an evil and corrupted soul tastes to a demon. It's something they cultivate like a viniculturist his vine. And it is a delicate art indeed to harvest the tainted soul at it's peak, either at a moment of regret or just the moment before they got too powerful. This first post about the machinations of evil will illustrate how the weak may be corrupted. It's system agnostic and could be used with every system that uses ability scores and xp for level advancement. Be warned, this will give the game a very dark turn*. So: magic users may become witches or warlocks by selling their soul. Her are my thoughts.
A human** magic user may sell his soul, if the opportunity arises. The decision alone will shift a lawful character's alignment towards neutral. If the requirements are fulfilled, they just gain the next level with all the benefits and powers associated with it (hp, spells, etc.). No xp needed. A character may start doing this at level 1 or on higher levels. If he enters later, he'll only have to pay the price needed for the located level. A demon may offer special powers for those prices not yet used up to level 5 (see below), so if a character is already, for instance, level 4, he might buy special powers instead of the levels he already has. If a demon sees a possibility to get away cheap, he will do so, of course.
The Lower Price Ranges
and Special Offers (enter at ANY level!)
Notes: A demon will naturally never talk about the consequences or the fine print. It's always just "I really only need the memory of your first kiss to make this happen! You won't need it anyway and you'll be so much more powerful afterwards...". Always leading a character nicely in the wrong direction to screw him up. And even if a character is only using it to gain a one-time benefit, the demon just won't let him alone and the dark deeds accompanied with it will fall back on the character somehow, someday.
A pleasant memory - shift alignment towards chaotic, if this shifts the alignment beyond chaotic, the player needs to succeed in a save vs. death or he looses control over the character (who is then considered "evil" and going for the next 1d6+2 levels in this list), gain level 2
Special offer: By giving the entity a pleasant memory, a character gains the ability to always float on water instead of drowning. If that isn't enough a demon might add a spell slot more or two. The character will still shift alignment.
A first kiss - loose 1d4 points CON permanently, gain level 3
Special offer: Giving away this memory will make the character immune to the effects of fear. He'll still loose CON.
Ambition - the character has sold his complete soul now and may only gain xp when following the orders of his demon lord, gain level 4
Special offer: The character gets the ability to control undead as a cleric would. Of course he'll only gain xp when doing his demon lords bidding from now on. Now the gloves come off...
An unholy union - gives birth to a demons child, this is gender indifferent (be creative), gain level 5
A friend - kill someone that trusts you, gain level 6
A virgin - sacrifice a virgin (nothing fancy for the ritual needed), gain level 7
A child - kill it, let it feel the pain, gain level 8
An unborn - get it out of the mother, make the mother realize what happens, kill both, gain level 9
Build unholy shrine - needs the sacrifice of at least 20 hd of some innocents, half of them need to be consumed, the character will have an uncanny appetite for young human flesh after that, gain level 10
The Fine Print
Every level gained like this will have the character loose (1 + half the level gained) points Charisma. If this is reduced to 1 the character is considered hideous (with all the attributes you'd give a witch, nose, warts, cackle and all) and further losses will instead reduce Wisdom by 2 points per further level gained this way. After reducing Wisdom to 1, the character ages 10 years per level gained. They won't die because of old age, though, they'll just look incredibly old, fragile and ugly...
An entity powerful enough to grant this powers won't allow the ones it granted those powers to become more powerful than they are themselves, of course. They'd rather arrange an accident and claim their price. Sometimes the victims get wise, though, and fight back. Best way to do that is killing the demon lord that owns your soul, gaining it's power in the process. Betraying the demon also has a nice ring to it. Someone like this might become a demon himself (it has to start somewhere, I guess). Other, more creative solutions are always possible, too.
Up to gaining level 5 a character sells only parts of his soul. If he gets the complete package including level 5 he has sold his soul complete. Being already level 5 or higher, means he'd have to sell his entire soul to gain the next level. A character level 1 that wants to buy any other level but the next, will have to pay for the levels in between, too.
If a character is able to recover his soul, he will, of course, loose all benefits he gained by selling it. The crimes he did will remain, though, and he is still accountable for them..
A DM or player might want to keep a separate tally for regular gained xp. For calculating regular advancement, the xp for the levels gained by selling the soul will count as if gained regular, too, and all other xp are added to this. So if a character has earned 2500 xp and bought 4 levels he have 22.500 xp and he'll need 17.500 xp to get level 6. If he would gain 5.000 xp more and decides nevertheless to buy level 6, his new xp tally would be 47.500 xp (as per the Rules Cyclopedia).
*Clarifications: If a player is foolish enough to sell his soul to a demon or devil, it has to have dire consequences. I'm not proposing this for the players to have fun "being evil", but to make them realize what "being evil" may entail. So this is to produce tension and maybe to see how far they'd go. But it's entirely possible too use this just for NPCs. In the end, it's a DMs choice what and how much of it he is willing to use in his campaign. My stance on this and further thoughts are found here.
**How this is handled for other humanoid races depends on the campaign world. Elves might have no soul to sell and all that.