Thanks a lot for the great feedback!
You make a really good point about him getting the skulls into his back. I didn't really think about it but i need to start figurine out such things if i ever wish to become a concept artist.
I actually loved the 3rd scenario, in which case the little demon gathers and drops the skulls into his back. It might sound weird but it's really interesting and unique.
However i will go with solution number 4. Having skulls protruding out of his skulls probably is a weird indicator of him powering up. Plus, can you imagine him at his full power. In theory he should have so many skulls that they could potentially distract from his actul design.
On the contrary i love the idea of him having horns grow in different sizes and places and it would allow me to create some great alternatives in case there are more of these demons in the hypothetical game.
I'm going to work on his evolution stages after i finish the other two demons.
Again, really helpful advice. Keep em coming <3