Hey Noro! Welcome to the sketchbook thread!
Awesome start to it too, nonetheless. The detail in your studies are great! But when it comes to your imaginative works, your rendering tends to look all the same. The cloth, the skin, the skulls, it reads as too smooth and doesn't really create any visual interest for the eye to catch onto. Do some more studies of different materials and understand how values work on each of them, and I think that will help you out.
Otherwise, keep on painting!