So he can change forms?

Not realy.
But then again.
I may play around with that idea. But nothing radical.

Well some form of mark that he always has no matter what. Like a birthmark or tattoo that could be used as a symbol for him being as he's a badguy and all they tend to have something that is trademark to them.

So some sort of mark the resembles the Xechestes along with the animal skull.

It could be anything. Just something that if seen people know he was there or it was him. Like a signature of sorts.

This is a great concept. It needs fleshed out though (yuk yuk yuk pun! *Is shot* ) No seriously though, LOL, He's got the beginings of something. He's terrifying and yet intriguing. Beware though of the monster trap. Many artists I knew fell into creating monsters that had no character and while this is ok and creative, you have a concept here that generates interest and the macabre nature of it begs an explanation. Is he miisunderstood good, deathly evil, or maybe represent something beyond those concepts? Is he dead or in his state or his world does his state occur naturally? The concept is a great start, now, flesh him out, finalize his design, and go further XD Cheers!