I've been working on a chaos since the release of the Fantasy 7th ed. army book. At the time, I was a much more competitive gamer. I sold most of my previous army to start over as I'd 1) become a much better painter, 2) my previous army was almost entirely cavalry and not really playable under that army book, and 3) preferred the newer models to those that had been out (although the original are still the best!). The 7th ed. list required a lot of infantry, so I based the army around 3 blocks of infantry, 1 undivided, 1 Khorne, and 1 Nurgle. I felt like the army standard bearer of such an army should be on foot as he would provide the most benefit to the rank and file.
However, I wanted something that looked suitably impressive--and the foot chaos hero model fit the bill perfectly. Say what you want about plastic models, but they make conversion so much easier. So I chopped off his spear, replaced it with a pole. Took the banner from the mounted marauder kit, and the head from the Chaos Knights, and voila!