Overall its a nice drawing - or maybe better say that each element is quite nice alone.
It feels like there are 2 or 3 different cultures that are gonna interact here, but the interaction seems a bit difficult in a drawing that aims to show each one of them together yet isolated.
If you are willing to make big changes, or maybe for future pieces like this, you can check some of "Dune" and "Star Wars" cover art on google images to get some references. Or any other book cover, game or movie poster that has some representation of multiple cultures/races.
The characters feel more or less equally represented and nothing draws the eye to a specific point (which can be what you want). You made the green goblin looking guy with a bit of a Triadic colour composition (green/purple/yellow), but the overall cover composition are the 3 main characters (I may be missing the stone skin looking ones on the back), so you may want to check your overall colour scheme, instead of focusing on the colour scheme of each character independently.
Would also take a look at the background again. You have a purple tower in the middle of a more yellow/brown set of buildings. If the idea is to use purple for shadows, so need to project the light and shadows on other parts too. Otherwise, why is there a purple tower there?
Other small detail for the lady holding the candle: the candle light is too white and the candle is right at the line of the other character hands, which causes a bit of a confusion there (for a few seconds I thought she was holding some part of his sword). That is what people identify as a tangent issue. Also, a candle alone wouldn't make those walls so bright.
Finally, just a question: Is it "A prince of the North" or "The Prince of the North"?
Hope this helps!