When Bartlett opened the diary on the desk, he finally realized that the high tower he had entered was not the kind of Isolation Castle that sealed treasures as he had originally thought, but rather a private residence in the Realm of Darkness.
As the skeleton Naga said, the owner of the residence had not returned since leaving for some unknown reason, which gave Bartlett an opportunity to break in as an uninvited guest.
Curious, the thief Bartlett picked up the diary on the table, looked around again without noticing any abnormalities, and began reading.
The diary was quite thick, and the yellow brass metal cover had three iron chains on it, but the chains were not locked. They were just tangled together in a somewhat chaotic way.