JASM is an assembler/disassembler for JVM bytecode. It provides a nice syntax for writing JVM classes in a bytecode-focused assembly language, and can also disassemble any Java .class file to JASM ...
The link takes you to the latest release on the Releases page. You may also wish to build it yourself, see Building for that. Darkest Dungeon uses a proprietary save fomat. The files have a .json ...