Again, without more pictures it's impossible to tell what is going on. I would like to see pictures of the 3 exposed exteriors of the fireplace chimney from the ground to the top. I would like to see what is going on where the chimney is attached to the house.
You have a crack on the corner. At this point we don't know what caused it. Something could have hit it or it could be the foundation is sinking or it could be a windstorm came along and blew so much pressure against the chimney that it caused the cracks. We just don't know.
From the looks of this, it appears there was a footing brought out of the ground when the house was built. The mortar looks like it is a slightly different color. That appears to be just below where this chimney crack starts. That footing also appears to be level in your photo..
So the first thing I would do is to put a level on that area below where the crack starts and see if the footing is straight and level.