Possible tags to help you out:
Magic
Pixel art
Spell book
Dark
Dramatic Illumination
Ominous
(Whatever program you used to make it)
ADDENDUM: “Is it strange to mark it with "pixel art" when it's already being submitted to the pixel art category?” Oh yeeeah, there IS a category for that! I tend to overlook it because it’s usually the tags the artist wants you to focus on and the category is just something they have to put.
Even with the category, I still see “pixel-art” as tag, and quite often too. So it’s not that strange: redundant but not strange.