Hey Nahgul! Great job on this! @Steve Lenze is spot on with the draw over. What I'd add onto it is that I'd try and organize your values more. Your edges right now aren't distinguished, and your values are a bit all over the place. Right now there isn't a clear focal point, which makes the entire drawing look a bit mushy. I'd recommend looking at Charles Bargue for separating values. I think if you improve your value separations, it'll greatly improve your drawings. Good luck!

I think the eyes should be bigger

Hey nahgul, Nice drawing from imagination, it's not easy. I did a quick draw over to show you why it's looking off. Just remember that the skull is not perfectly round, it has corners and planes. Also, add the center line and other guide lines to help place the feature correctly on the head. Hope this helps :)