Hi Typhon,
I like this idea for an image because I think it is interesting to have an undead creature image that does not have a gloomy or evil mood. Right now I think you are having difficulty because of your overall value arrangement. You have chosen a 'gloomy' setup for your scene by having a foggy, indistinct background, an undefined light source and an abundance of darker values throughout the composition.
This is a challenging project. I would suggest perhaps to make the background more like a real forest, that is inviting and appealing, to contrast with the ghoulish character. This would allow you to extend the value range and give you opportunities to add more vibrant color to lighten the mood of the piece. I made a small sketch to help explain my idea. There is a lot more directions that this could be taken; for instance you could add a vast rolling meadow in the background instead of a forest scene. The current arrangement of your background is more cramped and creepy, and this also adds to the 'evil' mood of the piece. Hope this helps and good luck.