Someone built an open-world combat prototype in Unreal Engine in just a few days, and Claude Opus 5.5 directed the work from inside the engine.

The detail that sells it: the grass burns and darkens wherever the sword lands.

How it came together:

01 / Opus 5.5 connected to Unreal Engine through MCP.

02 / The character generated in Tripo AI with Smart Mesh P2 for a clean low-poly mesh.

03 / Cleanup in Blender, then import into Unreal.

04 / Opus builds the map: terrain, hills, paths, autumn trees, flower meadows.

05 / Movement and sword attacks on top of Unreal's third person template.

06 / Sword trails added with Niagara.

07 / Grass that reacts to every hit, burning and darkening under the blade.

08 / A run through the whole world to fix what looks off up close.

The lesson worth stealing: effects should touch the world. A sword trail is nice, grass that remembers the swing is what makes it a game.

(Built by xikhar, all credit to the dev.)