http://theartinspiration.com/wp-content/uploads/2013/11/scary-horror-wallpaper-trees30.jpg
In the left of the image theres a strong black wisp of smoke that I’d like to recreate. Is it possible in Unity or is this something more that would have to be modeled?
To create something like that you would either need to make a super complex particle system, or draw the effect as VFX. There is no easy way to do it, but if you need something that looks like a line, you can always mess around with a line renderer.
The effect you have in the picture is most likely a simulated 3D render, which most likely could never run in real time due to its complexity on a consumer grade device.
Indeed in most cases you would just use animated texture to fake it. HOWEVER, Unity is currently testing a new feature to renderer similar effect, namely ribbonized particle trails, in real-time in a public alpha build which you can find it in this official forum thread. I attempt to quickly re-create the wisps of smoke in your reference, but there is some technical issue as you can see in my video which need to be fixed by Unity technicians.