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I never saved any of the previous stories, sadly, but someone archived the 'paladin's assassin sister sells her soul' story which can be read here:
As a summary, however, the erinyes had dominated a gold dragon and was intending to carve out a small kingdom for herself after escaping Hell, which is currently undergoing an upheaval in this setting.
The paladin attacked her while the gold dragon was raping the rest of the party, and she offered her surrender, which the paladin accepted in order to prevent either the other PCs or the gold dragon being killed.
After she surrendered, the wizard/rogue backstabbed her while she was being interrogated, so the paladin had to step in in her defence. Desperate not to go back to Hell, and also sensing a juicy target for corruption, the erinyes offered her services to the paladin, hinting that she could possibly be redeemed.
The paladin thought that was a hilarious concept, and after brow-beating her true name out of her to enforce some ground rules, accepted.
Since then, over various adventures she's been debating morality with the erinyes, who over time started acting in a helpful and selfless manner, even as she vigorously denied or rationalised her actions. She became Lawful Neutral after the story linked above.