Write an interesting, riveting, lengthy short story in the form of a crossover fanfic chapter of a fanfic of Fate Stay Night/Prisma Illya  and Eromanga-sensei with plenty of action, dialogue and exposition, in which Sagiri Izumi is an alias that Illyasivel von Einbern uses while out in public and Hana Umezono aka the famous light novel writer Muramasa Senju, is in fact an identity that Sakura Matou uses to move around in the non-magical world to avoid attention from rouge magic users and the Clocktower. Elf Yamada aka Emily Granger is actually Reines El-Melloi Archisorte, and her rivalry with Illya aka "Sagiri" and Illya's twin, Chloe von Einzbern, who is known as Megumi Jinno in public, is due to Illya's relative ignorance of the ways of the higher ranking families of the Clocktower. Sagiri locks herself in her room when she comes home from school because she is afraid her beloved adoptive big brother Masamune Izumi will find out that she has become a magical girl, and also to hide the fact that she and Chloe have incestuous feelings of intense romatic love for him. 

Of course, Masamune is clueless to the truth about his family or the fact that Hana and Emily are even magicians, being concerned with getting through the day. Sagiri on the other hand, is also trying to hide the fact that she is the legendary erotic illustrator "Eromanga-sensei" from Masamune, which Chloe/Megumi and Sakura/Hana both tease her about. 

The chapter ends with Masamune opening Sagiri's room door to check up on his adoptive little sister only to see Sagiri about to have a discussion with Reines as Emily/Elf, Sakura as Hana and Chloe about the mysterious happenings in Fuyuki. He is surprised that Sagiri even has a social life and attempts to close the door to give her privacy, but Reines as Emily uses the opportunity to try and one-up Sagiri, telling him that the girls were discussing how to confess their love for him. Sakura as Hana then secretly uses her magical powers to push Sagiri/Illya on a shocked Masamune, causing their lips to touch. Masamune realizes that Sagiri has been using a sarashi to bind her large breasts as he feels the weight of her huge boobs on his chest, causing him to blush as he tries to hide his raging erection. 

The door then closes, and the four girls then proceed to use noise canceling and notice me not spells to secure the room before proceeding to drag Masamune inside the room and then have passionate sex with the man they all love. Meanwhile, a mysterious magus watches the whole affair in confusion through the scope of a magically reinforced sniper rifle, the badly mutilated corpse of a rouge CIA asset that was trying to kill Illya/Sagiri and Chloe/Meguni lying beside him and the body of a Russian FSB agent that was just about to try and break into the the Izumi mansion lying dead near the steps of the front door. The mysterious magnus comments he will never understand the ways of the top families, while also stating that he wouldn't be surprised if Masamune and the girls end up having a lot of babies soon before continuing his unnoticed vigil, a shadow of justice unseen or unnoticed by the people he is protecting. 

It is strongly implied that the mystery magus watching over Masamune and his harem is in fact, a member of the side branch of the Emiya clan gone rouge, choosing to fight a one-man-war against criminal organizations and rouge magicians after getting dissatisfied with the Clocktower. 