By making an in-game bargain with an lower plane power, you can gain access to the Dark Host pseudo prc. In return for a significant stat boost (but random as to whether physical or mental stats are boosted) and forced alignment change towards evil, the player loses some control over the pc due to the influence of the being they host. The idea is similar in concept to the story of one community member's adventure (I won't say who since it would kind of spoil if for players of that person's adventure), at least I can say I had the idea independently.
This is an entirely optional bargain, it is not necessary to complete anything in the game, it is not given anywhere the player must go, and the player is warned in-game of the unpredictable nature of the bargain. Lance Botelle has a poll up now on predestination versus free will. Is your pc's answer to the lines below predestined, or free will?
PC: So I become just an empty shell for something. Somehow, that does not sound appealing.
Lower plane power: You do not become an empty shell. You... share, though of course you are strong enough to remain in control.
PC: Of course. Somehow I doubt you are entirely telling the truth.
Lower plane power: How much is truth and how much is lies is for you to decide...
Here are some examples of the loss of control: one quest has the player try to convince some local ruffians to take care of something so the player doesn't. A normal player can use appraise, bluff, or taunt. The Dark Host forces the player to use intimidate, as the being they host overrides the pc and speaks through the character. Another quest sees the Dark Host forcing the player into a fight they'd most likely avoid otherwise. It's not all bad consequences, a murder investigation the player is asked to undertake is instantly solved because the Dark Host "sees" the evil of the murderer, allowing the pc to instantly identify them.
There are currently around 20-25 planned spots for the Dark Host to influence things, eight are implemented so far. Players will generally be given the Dark Host option late in the game, so they probably won't see them all. Exactly how many a player sees will depend on whether they get the physical Dark Host, or the mental one, as the respective beings have different approaches to their evilness. The physical one is more of an intimidating brute personality, while the mental one is more of a conniving knowledge dealer. The Dark Host dialog options are unique, so if a non Dark Host player can use intimidation during conversation, the Dark Host intimidation the player must choose has a different line. The Dark Host dialog lines are umm, appropriate to what happened to the pc. People may not like their lines, but they did make the choice that allowed it. I had complaints about how you could do and say evil things in Path of Evil (yes, I know, I warned people of that in the campaign description, I still had complaints), and Dark Host is worse
I'm torn about giving the player an opportunity to cure themselves of this state. One possibility, the player tries, but the cure automatically fails (predestination!), and the player is stuck with it.