Starting a sentence with "unless" is much the same as starting a sentence with "because." Although our 4th grade teachers told us NEVER TO DO IT (because in 4th grade it often resulted in a fragment) we can write a complete sentence starting with "unless" or "because."
This will most likely create a complex sentence (one independent clause and at least one dependent clause) that begins with a dependent clause. This means you must use a comma - and add the subject and verb before you finish the sentence. (It could also create a compound-complex sentence if you wanted to add another independent clause.)
Unless both a subject and verb are used after the comma, this sentence will not be complete.
Subject: this sentence
verb: will not be
dependent clause: Unless both a subject and verb are used,
If you said the first part of the sentence by itself you would be asking, "Unless both a subject and verb are used, what?"
Here's another, more simple example:
Unless John goes, Sue will go to the concert alone.
There is an argument that before the comma contains a subject and a verb: John and goes. It is the presence of "unless" that makes that part of the sentence dependent on the rest of the sentence.
Now - in fiction, we might actually read these two sentences separately like this: Sue will go to the concert alone. Unless John goes. Technically this is sloppy - but in fiction we'd understand it and call it a "style" choice.