Can you tell me please what you think about my head and how i can make back of this head more , cool ? Back of my head under Thorn's Crown is just cobra color :c

You could make a creepier look to it, like putting a eye on the back and maybe removing that tribal looking crown and doing better details in the eyes and better shadow.
You also could make something on the top, like a hair or something like that.
The head looks cool, but it just need more work

I agree, that tribal looking crown with black color looks kind of disturbing, it doesn't really suit with the main style and color of the head, and also killing the style of the head in my opinion.

Maybe adding a creepy looking shit with the same shading style like the front-look? adding a hair won't hurt too, or maybe some realistic looking scars? well, it looks better than tribal-looking stuff like that black crown tho.

I will try
Thanks for advice

Not going to say much but PLEASE!

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I cannot stress enough how mirroring heads messes with shading and lighting. I will ask of you one thing; whether you chose to listen or not is (of course) 100% your prerogative:

Mirror ONLY the sketch and color & DO NOT mirror shading. Shading, unless you have enough experience in making textures that you can do whatever you please, should be done last and naturally. That way, shading doesn't isolate itself in the middle of objects or overlap into a weird shaped imprint on the face.

Thanks and I'm out!