The squad, like "Ferrugo" is pretty straight-forward. A bunch of dudes carrying normal weapons and some shields to represent the increased toughness and Feel no Pain of Plague Marines.
The Chaos Lord is less transparent. The backpack is meant to give him status and longevity (it's the backpack from the old Iron Warriors Warsmith model) while the sword and gun combine to represent the Tzeentchian Daemon Weapon. The entire reason I took Mark of Tzeentch with a Daemon Weapon was because I liked the idea of modeling the ranged attack with an exotic gun of some kind. I figured that an Iron Warrior that had served the cause over the course of millennia has probably come across and/or tinkered up a powerful gun or two in that time. I think the gun used, from the ForgeWorld Boarding Assault set, has just enough sense of the exotic to represent that while the big-ass sword is an additional reminder that the Daemon Weapon also serves in close combat.
I know it's a sword from the new Blood Angels kit, but the distinct lack of blood drops make me think I can pull off giving it an Emperor's Children vibe in the painting. It was a gift to Gibeon after a particularly hard-fought compliance during the Great Crusade. Somewhere out there, there's an Emperor's Children commander traipsing around with Gibeon's old master-crafted plasma pistol.
Currently on the workbench I've got the remaining 16 models from the Polybolus squads. If I can get them done this weekend, I'm in the home stretch. Just one tricked out Rhino, two magnetized Dreads and two mostly stock Vindicators to go!