This item is found in the Elona+ mod of Elona. Mysterious diaries can be made using a pot for fusion or a pot of alchemist. Recipe is available by default. The recipe requires any summoning diary (including Mysterious Diary itself), and another book. The book can include another diary, red or blue books, but not spellbooks. The ability requirement to make it is 10 in literacy. The time required is listed as 10 turns and 1 hour.