Diary

Ren was familiar with this particular box, this was a box that was created during his rule. It was created for the daughters of demons with high social standing. These boxes were meant to be a toy and form of training for young daughters. 

It was a storage device that would only open when the right amount of mana was used. This was not only a popular toy for practicing mana, but it was also a nice place to hide one's belongings. The box was crafted using the bones of a monster that could deflect almost all physical attacks. It was also imbued with a spell that makes it hard to open for those that aren't bound to it. 

Still, for someone of Ren's level forcefully opening the box wasn't a problem at all. When Ren forcefully opened the box he saw that inside it was a diary. The diary was made from the skin of a manticore which meant whoever owned this diary was the daughter of a very influential demon.  

Ren opened the diary and started reading a random passage.