These are thumbnails for a Lich. Liches are undead creatures formed when powerful people magically bind their souls to an item in order to ensure their immortality. Said immortality doesn't affect the aging process however, so they all eventually end up looking like your random zombie/skeleton dude, except with some pimping armor and weapons.
It was incredibly fun doing these thumbnails, something that I choose not to do sometimes because of time constraints. It also makes sure you only put out your best work, because a lot of the really awful and unoriginal stuff gets left behind in the sketchbook. I can't say I'll be very rigid about doing thumbnails for all of my future work, but it's something to keep in mind.