Squire James said:
My "dark side" suspicion is that this was a Plot Railroad, and there was little the PC could have done to avoid the situation, Will saves or no. If this were the case, you probably wouldn't be asking us what to do next, because you would already have the Next Step plotted out!
While I've been around these boards long enough to recognize this as a valid possibility, I'm going to have to plead my innocence here. The encounter with the Erinyes was planned, and she WAS going to profit from it. But how became a question. If the party refused to deal with her, she would have simply gone to the pack of devils chasing them and sold the party's location for a soul or two. The party would have evaded the devils (or not) or fought them (or not) and whatever would have happened could have happened.
The Erinyes only asked for a kiss (and therefore a means to try and Charm someone) and any party member could have supplied it. Only the Rogue/Royal Explorer was bold enough to give it a try (somewhat of a poor choice given his low Will save). The Cleric bestowed his Protective Ward ability on the PC giving the kiss and it preserved him from her Charm attempt.
She still wanted his soul and was persistant in extolling the quality of the information she had as well as touting her tremendous sexual skills. Finally her (entirely non-magical) seduction worked and, lured by desire and feeling like his chances were good given his earlier success, the PC went willingly to her (enormous) bed. He needed to roll a 13 to make the save. He rolled a 12. He even spent a "Fate Chip" to roll again and rolled a 7.
I'm not sure how much more free will I can give somebody than that.
Nonetheless, I still don't think that he immediately becomes Evil for that alone. It wasn't as though she said, "If you sell me your soul, I'll strike down your enemies, make you rich and Enlarge your private parts." and he said, "Yep." He was somewhat of a willing dupe who agreed to do something questionable (ok, VERY questionable) in order to gain some information that would help other people a lot more than it helped him.
I think he becomes Evil when he attempts to regain his own soul at the expense of others (i.e. when he uses the dagger).
And Epimetheus and Kurzak T, I've read your ideas and I like them a lot. I'll keep these in mind when I do the final design on how the dagger grows in power.