Weird title I know.

Does anyone know any quality literature where the protagonist has an evil/dark being living in their head that is only visible to themselves.

Could be a fallen angel, demon, or some other similar entity.

 

The Dresden Files :awesome

 

I guess Fight Club sort of counts, with

Spoiler

Tyler Durden.

 

Already read/watched those.

What is there absolutely nothing else?

 

Though it's been twenty-plus years since I read them (and my memory of the plot is a bit hazy), the Dragonlance books might qualify with the whole Raistlin Majere/Fistandantilus thing.

 

Rand al'Thor has Lews Therin living in his mind in the Wheel of Time series.

 

Not really. Not only was it a tiny plot-point it was more like Fistan...something was actually a part of Raistlin in a weird way. I can't get too into it but there was no actually distinct entity or meaningful change.

 

Death Note.

I had one more in mind, but I forgot. Will edit this if I remember.

 

When looking for fiction with a certain characteristic, TVTropes is probably your best bet. But maybe you're thinking more Enemy Within, or any of the other similar-but-different tropes. I could've listed a few from there, but why not cut out the middleman.