UndertaleModTool
The most complete tool for modding, decompiling and unpacking Undertale (and other GameMaker games!)
Overview
Direct modification and analysis
Has editors for most GameMaker assets, code, and parameters. Includes GML VM compilation and decompilation.
Extensible and scriptable
For features not natively supported by the tool’s interfaces, C# scripts can be used, or code can use UndertaleModLib directly.
Accurate data loading/saving
When not making modifications, every last byte of data files are tested to load and save with 100% accuracy, for supported games.