Pandora Behaviour Engine Plus is a C# desktop application (Avalonia UI) for patching Skyrim's Havok animation behavior files (.hkx). It parses mod-provided patch instructions in Nemesis, FNIS, and Pandora XML formats, then injects changes into base behavior graphs to support custom animations. The architecture follows a modular DI pattern with services for mod detection, plugin loading, path management, and engine execution. Key components include a state machine for engine lifecycle, a plugin system for extensibility, and a set of reusable change types (IPackFileChange) for applying edits to Havok packfiles. The application provides both a GUI and a CLI mode, and includes extensive test data (hundreds of patch files) for validation. While the core patching logic is solid and well-structured, there are signs of technical drift (preview .NET target, some dead code) and the large test data set presents maintenance overhead. Overall, it is a strong, professional-grade modding tool with reusable patterns for behavior graph manipulation.