Mordrak is able to field a unit of Ghost Knights when he appears on the battlefield due to a psychic manifestation of his survivor's guilt. This particular grey knight is a justicar of his Ghost Knight retinue for a small army that I'm constructing (whilst waiting for codex: chaos legions to make an appearance, of course...).
The miniature is a standard grey knight terminator, armed with a halberd and adorned with various paraphernalia that we have come to associate with grey knights (ancient liber, etc.). Instead of a regular helmet, this terminator sports a skull. This is inspired from the Ghost Knight status -- although the rest of my Ghost Knights will have sealed helms, I wanted the justicar Ghost Knight to stand out a little bit. Hence I clipped a skull from one of the chaos vehicle sprues and carefully glued it in to place. Although the skull lacks a jaw bone, it feels about the right size for the cavity where the helm should be. I just need to figure out how to cunningly paint the chain that I've left attached to the skull so that it is not too visible (the chain was left in place to give the skull a bit of height from the neck of the terminator armour).