Head is an orcs head with a black orc facemask. Right arm is an Imperial Guard Lascannon cable with an orc hand on the end (looks painfully like exposed muscle in person) and a Tyranid warrior's carapace spike for the shoulder. The left arm is an Ogre's arm with a gauntlet weapon. The growth on the back is a adrenal gland from a Carnifex. The feet are Smoke Launchers from an IG Chimera and an old SM Rhino. The legs are Orc Boy arms. The inside of the torso is made of a Cadian torso with an ogre gutplate on the front and back. And yes, the front face is from the Mordheim building sprue. One of the few parts I have left from the box set I bought way back when the game came out.
My goal was to make it look like it had a form that was being violently ripped apart and new things sprouting out. For the Onogal / Nurgle I wanted to have open sores and pox and pustules. For the Tchar / Tzeentch I tried for avian qualities as the Blue/Purple feather is going to be a theme in my warband, at least for heroes. Also I tried to make the feet look like birds feet. I also thought the should spine popping out quickly was fitting for the Great Mutator.
So far this guy has not done much other than absorb a huge amount of arrows and tie up some Youngbloods.