a lot of things...
First, always post a TGD of as far as you have gotten. So we know how you are trying to do things (there are several ways to do almost anything, and they are not going to be equal).
For example, of the possible ways to do what is in your image, the two that make the most sense in my mind are a displaced cube or a imported OBJ. I would prefer to sculpt this, but a cube would be good too.
Plus, if someone is feeling very helpful, thay may want to build something for you, but you should not make them start at zero. It would be easier and faster if you already have a start for them. (that kind of help does happen in this forum a good bit, just not all the time
)
After you displaced a cube to this shape (which in its self is going to take time, and require a lot of practice), then you can worry about the "hardness" and look of the stone.
Use a strata node to get you a lot of the way in terms of, well, the obvious strata. Then, there is "cubic noise" clip files by user "mogan", and that should help you the rest of the way. (but honestly, that will not be easy for a new user, its not "easy" for anyone)
Hardness look will also be helped by color shaders, and dirt clips in various places after form is complete.
I don't want to talk you through the steps for the basic project without seeing where you are on your own. You could be closer than you think you are, or really going about it in the wrong way. Hard to guess. Post your file. (.TGD)
Also,
Have you seen this yet?
http://geekatplay.com/terragen-tutorials.phpYour image is not a "basic" level TG project IMO, due to the specific shapes. But you should know the basics before you start in on cubic noises and rock spires. (Just making sure you know about the resources that are available for you)
IF for some reason you need this now and don't have time to do tuts, then clip files are going to be your best friend on this.
