Terrain Importer

This mode allows you to create sectors from a Wavefront .obj model.
Menu path: File -> Import -> Terrain from Wavefront .obj.
Action category: Tools.
Default key: none.

Usage:
Create a terrain model in your favorite 3d modeling app (Blender, Earth Sculptor or any other, which can save a model to Wavefront .obj format)
Use File -> Import -> Terrain from Wavefront .obj action to import it as sectors. Each polygon in your model will be transformed into sector.

Limitations:
The model should be triangulated.
Polygons should not overlap each other when viewed from the top.
UDMF only, because the mode uses vertex height offsets to create sloped floors ("zfloor" vertex field).