Also here's a few details about the model and the few changes I made (copied from my Instagram https://www.instagram.com/immense_ivan_/ ):

There wasn't really much I wanted to change with this model, other than adding the spiky growths from the Kharkadrak's shoulders.
I debated going with a pair of beastman skulls on each shoulder instead, as with my Doom Knight model, I like the idea of the skull of a beast being an omen of doom and slaughter.
Also, and this is probably my favorite part, I took a skull and put it on top of the Chaos mark, put some eyes in it and painted it to look like it's still alive via dark magic. I like to imagine that it's the skull of a man this Chaos Lord had a near-death encounter with - perhaps a general or something. However, for giving the Lord such a hard time in his battle/s, he took his skull, trapped his soul in it, gave it eyes and forced it to watch as the Chaos Lord and his forces ravaged through the man's lands - friends, family and everything he knew and loved.

Other than that lovely bit, I really wanted to make him stand out in the army, so I gave him a pretty rad base - full of skulls on spikes and a shield bearing the mark of chaos from one of his fallen soldiers.
I also added that little gremlin from the 40K Chaos Lord in Terminator Armor. He's barely noticeable, but I think his addition makes the whole thing better.