Satan's Diary (ConceptTopic, 43)
From Compile Worlds
Satan's Diary (and not Satan's Dairy, mind you) is a diary authored by Satan. There are multiple iterations of this book, most notably the one appearing in Madou Monogatari (Sega Saturn), but there is also one inside a "secret chest" in Satan's Tower near Muffin Town.
Madou Monogatari Version
The contents of this iteration of the Diary explains how Yoggus and his Yog selves were able to enter the Madou World. Satan reveals through the text that he was bored with the world he'd created, so he had simply decided to portal Puyo from another world to mess with Arle Nadja and the gang again. Instead of summoning Puyo, though, he reveals he had accidentally let Yoggus and Lagnus in by opening the door to their world at a convenient time. Yoggus split up and vanished. Satan then had to stay in the tower and deal with the spacial distortion Yoggus caused while hiding Lagnus' body in the Crystal Prison to keep it safe. He also wonders if a certain "she" will eventually notice the chaos, but the translation sheds no light on the matter, as the text is left vague. (Chao speculates the she to be Eldora though.) Once Arle, Rulue and Lagnus defeated the 8 Yog selves, Yoggus emerged from the spacial distortion below his tower. Satan, not expecting this, had left the diary in a conspicuous place where Arle would read it while he dealt with holding Yoggus back with all his might.
Muffin Town Version
The Muffin Town Diary chronicles some behind the scenes action of Satan's Silly Plans™ in Compile Worlds 2. Most notably, it describes his frustration in Arle Nadja's rescue by White Feli as he dispatches his minons Draco Centauros and Jan to deal with it. He also goes on about negotiations with Angol Mois involving removing the Neo Angolian infusions withing Lord Marcus' body, who holds Marcus for ransom after making "interesting discoveries". Once he realizes Jan and Draco have left their own little scribbles in the book, Satan hides the book in a chest and stops writing in it after he finishes negotiating with Angol.