The Lich is seriously the perfect villain. No redeemable features, utterly devoted to his goals, and freakin' terrifying as heck without needing to put in any effort. Are we sure Adventure Time is a kid's show?
Two things, and not trying to come off as a dick, but his horns are curlier than that and his pupils are pale green. With the darker overall tone of the image, I'd go with an electric green for the eyes to make them seem somewhat 3D, lending a more otherworldly tone to his appearance.