Disclaimer: I don't own How NOT to Summon a Demon Lord. It's written and illustrated in the real world by Yukiya Murasaki and Takahiro Tsurusaki in that order.
A Demon Lord's Bizarre Day
Peace of Mind Inn, Faltra City, Kingdom of Lyferia
"In retrospect, Diablo, you pretty much brought this down upon yourself." Rem said a bit too smartly for the sick Demon Lord's taste as he remained immobile on their inn bed with a groggy expression.
"Please." Diablo groaned. "You make it sound like I wanted to get sick."
"Uh, he's not wrong there, Rem." Shera intoned while Klem and Edelgard looked on in a mix of both concern and disgust.
On a prior quest for the Adventurer's Guild, Diablo had chanced upon an unknown apple-like fruit. In the mood for a snack, he took a bite out of it, but was unaware that it was poisonous to Demons. Upon getting back to the city, he had vomited, sneezed out gunk, and shown a bad case of diarrhea which brought no end of cleanup for Mei to do. From what they could tell, Diablo wasn't in danger of dying, but it had been a stressful day regardless.
Klem shook her head in the negative before declaring. "Okay, that's enough! Diablo, I'm curing you right now!"
Before anyone could get what she meant, Klem placed her hands on Diablo's forehead and brought out her Demon Lord powers to cover his entire body. By the time she was done, the latter's sickly form was replaced with a healthier appearance.
"Diablo, are you okay?" Shera wondered in curiosity.
Standing up straight on the bed, Diablo touched his forehead to test his temperature, and found that it was normal. "I feel pretty great!" He cheered before patting the smiling Klem on the head. "Way to go, Klem!"
Before everyone could celebrate properly, though, they soon saw that Diablo was turning smaller until finally, all that was on the bed was a Demon baby with Diablo's hair color. As the girls in the room stared at the infant with dropped jaws, it didn't take long for the kid's stomach to make a rumbling sound and then cry his heart out in hunger.
Edelgard's observation was right on the spot when she said. "This problem just got worse. Didn't it?"
Holding her placating hands up, Klem defended herself. "I didn't expect this, just so you all know!"
"I'm more worried about who'll be feeding Diablo at the moment." Shera commented with a drop of sweat.
Folding her arms and doing her best to be nonchalant about it, Rem replied. "Well, don't look at me, Shera. You're the one with the biggest "assets" to help out here."
Shera couldn't resist holding her hands over her chest in embarrassment at the statement.
Got the ideas of a sick Diablo and later a regressed into a baby Diablo from AnimeLover20 on A03. They each seemed like a good enough idea at the time, so I merged them together and this came to be for the sake of expanding this series' fanfiction roster, of course. Wanted to make this a bit more romantic, but decided to make it general instead for the sake of time management. I've got other stories, short and long, to do after all.