Here's a book cover I had to do for my class. I chose The Silmarillion by J.R.R. Tolkien. The scene depicted on the cover is the battle between The Balrog and Glorfindel.
This assignment easily qualifies as the hardest one I have ever done. I had to deal with Balrog anatomy, fire everywhere, snow, snow that reflects fire etc. And on top of that I struggled to find a design I liked.
Edit: Repainted Glorfindel. He was way off from my drawing. I don't know what happened. Oh, wait, I do. Sleep deprivation.