Question about terrain

You don't need a plugin - what you need is a skilled modeller and texture artist.

The unity terrain system is a fairly vanilla (read: bog standard) 2.5D heightmap terrain system -- it simply displaces a plane of subdivided mesh upwards; what you're after to achieve what's in that image, you have a number of (limited) options:

1) A voxel based terrain system. A number of these exist in plugin form eg:

https://www.assetstore.unity3d.com/en/#!/content/8131

https://www.assetstore.unity3d.com/en/#!/content/8176

https://www.assetstore.unity3d.com/en/#!/content/9043

https://www.assetstore.unity3d.com/en/#!/content/9180

Bear in mind that a voxel terrain is a very complex system compared to unity's; I've worked with a few in the past, and I can say it's a very hairy and complex setup prone to issues... however, some of the above look pretty slick!

edit: adding option #2... 2) Hybrid unity terrain + detail mesh. It's now exceedingly rare in professional development to use a height map terrain as the dominant visual element in terrain rendering... usually we just use it as a base, then create a construction kit of interesting mesh to populate it with. These "interesting mesh" props are usually the main visual element and can be HUGE. Check out a recent game such as Dragon Age Inquisition and you can barely spot the heightmap anymore, it's covered in so many mesh plug's :D

3) pure mesh - modelled, uv'd and exported from your modelling program of choice. Collision is the first problem maker here - you'll need a mesh collision component, ideally simplified enough to not introduce a huge amount of performance overhead. Second issue is usually complexity for large landscapes, you usually need to invest time and effort into a chunk based lod/streaming system.

Hope that helps!