Did you intend the dark gray border around the edges to be part of the cover design? If so, a border like that would create noticeable irregularities on the covers not trimmed perfectly (which would likely be most of them).
This isn’t something the designer would typically decide, but a book like this would benefit from a subtitle providing just a bit more information about the book. I’ll also mention that front book covers can’t be considered as something separate from the spine and the back cover — they all need to work together.
I likely wouldn’t have the tip of the sword sitting precisely on top of the monster’s head. It creates a small focus of attention that’s a little odd. The single point of light in the illustration — the monster’s eye — is another one of those things that draws attention to itself. The placement of the moon directly behind the sword’s handle is interesting.
If it were me, I’d probably selectively lighten up some of the illustration and introduce some subtle tonal differences in the colors so that everything wasn’t just a solid, monochromatic purple. When I squint my eyes just a little bit, the cover (other than the typography) turns into little more than a dark, solid shape.
Despite my comments, there’s a lot that I like about what you’ve done.