Mainly you have 2 working option. To make it with the engine's terrain tool if it has, or making a highres mesh using a sculpting software, baking a and using the baked maps, including displacement map on a plane, or you can make lowpoly with hand , but I would say Its a waste of time when you can import a heightmap. Terrain tool and displacement map combo can work too. Rocks/stones and other decorations can be separately imported meshes.
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http://wiki.polycount.com/CategoryEnvironmentTerrain?highlight=%28%5CbCategoryEnvironment%5Cb%29
Mainly you have 2 working option. To make it with the engine's terrain tool if it has, or making a highres mesh using a sculpting software, baking a and using the baked maps, including displacement map on a plane, or you can make lowpoly with hand , but I would say Its a waste of time when you can import a heightmap. Terrain tool and displacement map combo can work too. Rocks/stones and other decorations can be separately imported meshes.