Screw with her a little by ordering her to get Darkness off.

After Megumin straightened up again, she looked at Kazuma curiously. "So what next Master?" she asked.

"Actually, I want you to finish what you started," Kazuma remarked.

"What do you mean?" she questioned, blinking.

"Get Darkness off. You're free to do whatever you think will do the job, other than interfere with anything I am doing, if I decide to join in," Kazuma said. "If you give up before she cums, or I tell you to stop, I'll give you a sexual punishment, but a reward if you either succeed, or go until I tell you to stop. Darkness, do anything Megumin tells you unless I say otherwise."

"Yes Master!" they said at once, and Megumin resumed licking and fingering Darkness, getting a bit frantic about it as Kazuma chuckled.

"Anything?" Megumin asked hopefully as she flicked Darkness's clit.

"Nada. Well, kind of, but it's more like a normal sense of touch, no pleasure," she told Megumin, who huffed.

"Well alright then, get on all fours!" she told Darkness, who nodded, and did as Megumin ordered. "You're a masochist, so..." She proceeded to start slapping Darkness's ass.

"Nope, sorry!" Darkness remarked, confusing the hell out of Megumin, who slapped her ass harder. Still nothing. Giving in to her frustration, Megumin went and grabbed her staff, wound it up, and swung it, creating a resounding SLAP! on impact. Still no reaction from Darkness, even though Kazuma winced. If he didn't know Darkness had absurd durability, he would have vetoed that plan.

Megumin groaned and tried it again. And a couple more times, before she set the staff down, pondering her next approach, Darkness giggling all the while as she waited to see what Megumin might do next to add to the pleasure she would experience at once.