Fuck Darkness and tell Rin about the Note.

"So Darkness, you didn't get to finish, did you?" Kazuma asked her, and she brightened up.

"No Master, I didn't," she replied.

"Well then, come onto my lap and ride me," he invited with a grin, and cooing, she immediately moved to mount herself onto his lap. She guided his cock into her pussy as she lowered herself onto him.

As they started fucking like that, Rin looked over at Kazuma. "So... How did this happen?" she asked the three.

"How did what happen?" Kazuma asked through his moans as Darkness bounced herself on him.

"The whole sex slaves thing," Rin asked curiously. "I mean, if you asked, I would agree, but I know most wouldn't," she added, the lack of limits apparent in that statement.

Kazuma considered, before nodding to Megumin, knowing Rin wouldn't get upset with her desires released. "Oh, Master has a magic notebook called the Sex Note, given by a goddess of sex," Megumin spoke, even as her attention was mostly on Kazuma and Darkness. "He can make anyone do anything sexual. Darkness got it first, got him to enslave her with it, and then he came after me and Aqua. We're pretty happy about it though."

"Eh? I know magic is a thing, but that seems a bit far fetched," Rin blinked, noticably not angry at the idea, but more skeptical.

"Eh, Master will prove it in a second, just give him a second to finish up," Megumin remarked. As if on cue, both Kazuma and Darkness moaned out as Kazuma filled Darkness up with his seed.