In 2010 I created a custom level using the game Titan Quest: Immortal Throne.
Created a backstory and designed the layout.
Terrain sculpting and painting, asset/fx/sound placement, custom lighting.
Some scripting to unlock hidden sound assets.
All stock assets, since the toolset doesn't allow importing meshes or textures.
If you have Titan Quest: Immortal Throne, you can download the level here (20MB) to try it out.
The exterior of the level, in the editor:
A bird's eye view of the cave interior:
The center of the map:
The shrine in the cave:
Some sun coming in where the cave roof collapsed:
Where the water enters the map: