The characters come home one night and find a woman in their apartment,1 in normal clothing. Her message: “Your aid is needed for a quest.” She will say she represents higher powers of evil, and needs their help, since they have “so proven themselves in their other endeavors.”
She is actually an Erinyes, a devil from Hell’s second plane2, but she is indeed telling the truth. AC2, MV6/21, HD6+6, hp 36, at 1, D2-8, SA: poison (save for fall unconscious for 1-6 rounds), SD: see MM) armed with a dagger.
Her story is this: “A gem has just been found near here, which could have dire consequences for all evil persons. It gives Lawful Good people great powers when fighting evil.”
This is almost the truth—it only gives power when fighting demons or devils.3
She will attempt to get them to help—offer them 4,000 gp. Each time they attempt to bargain, she will go down 1,000 gp. If it goes to zero, she will thank them for volunteering to do it for free. If they still refuse, she will use her rope of entanglement and attempt to bring them back to hell with her. Note that she will state that she will give them the money when they give her the gem.
She will travel with them to the dungeon if they ask, but will not take more than her share of the fighting.