Not too far into the Deva Fortress level, you’ll gain the feather artifact to lift the bell out of the way to proceed. Continue forward and up the stair to the right. Continue straight while upstairs then left into the castle. There will be a bookcase on the right as soon as you inside. Check the left side for a book to interact with. Enter then go down the stairs to interact with a small chest in the table with a skeleton leaned over it. There will be a battle with four weak skeletons and one boss with around 220 hp. Win the battle and take the treasure. The battle wasn’t tough with my party at level 8-9.