Not too long ago I picked up an old converted Lord of Skulls from Glen (wrythsprimarchs.blogspot) but wasn't happy with the 'tank' section of the model. Removing the torso and pairing it with a set of Imperial Knight legs from bitsandkits allowed me to build a huge Khornate walker for my World Eaters army.
With this setup I figure I can run him as either a renegade knight or a lord of skulls. And I've since found out I have accidentally built one of what Forgeworld have called a Kytan, so I guess I can run him as one of those too.
With the removal of his huge tank-like lower section I had no way of mounting any of the large characteristic blood tanks from the Lord of Skulls model so opted to use a fairly high number of smaller tanks (9?) to help show what juice Khorne runs his engines on.
The trickiest part of the whole build was getting him a melee weapon. The original conversion didn't include the amazing axe from the Lord of Skulls model, and no other weapons I could access were big enough for this guy so I was forced to start almost from scratch. Using the existing hilt I built him a knight sized chainsword out of plasticard. It isn't perfect but it gets the idea across.