Love where its going so far bro! I love the axe's shape. Its like the love child of a chopper and scythe.
I think like Burak said, you should add more to the silhouette. Since it looks like a Kingdom of the Dead weapon, I'd go with a few skulls or parts of them.
For instance, where the axe has bone holes in it, there could be jawbones overlapped on top of each other up the back. Also some metal reinforcements on the handle where the two bone segments come together would make it look stronger.
They didn't say the skulls had to be human, so check a few refs and make your own up
Loving the color composition and shape already, keep it up bro!!