Not sure how this will work though and where to start, so I thought I would come here for inspiration, colour schemes and conversion ideas and how I would adapt the codex and allies???
This is what I've been doing for a while, and I have a few pointers as to what I've done:
- Chaos cultists are excellent for this. If you can nab some from Dark Vengeance, cool, if not, you still have access to the more limited selection in the cheap 5-man box they sell. Ones with autoguns make good guardsmen, and ones with autopistol+ccw are nice sergeants. If you have duplicate cultist models, I would personally swap out their weapons or heads for other bits and the like to make them look less similar, but that's not essential.
- Kitbashing Fantasy kits into IG. I have mashed together a mix of Catachans, Cadians, Beastmen Ungors, and Empire Flagellants in order to get a mass of mutated, fanatical infantry armed with lasguns, etc...
- It shouldn't be too hard to add spikes to some tanks or the like. I myself am just painting my tanks black, so they still fit alongside the Death Korps minis I inherited from a friend while still looking suitably "Chaosy," but you don't have that limitation Go wild with green stuff daemonic possession of vehicles, etc!
- If you want to make a slightly less competitive--but very fluff-accurate--army, consider bringing a bunch of Primaris Psykers and Wyrdvane Psyker squads who roll on Malefic Daemonology to represent daemon-possession among their army's numbers. Sure, any time they try to cast a big spell (and often when they try to cast a small one) they'll Perils, but that just represents reality tearing itself apart as the daemons who are possessing them hack their way through the walls of reality and make their presence known.
Then you have to deal with the fact that the armies are "come the Apocalypse"--there's a chace the guard will be too busy quaking in their boots or falling to the ground in worship to do anything the next turn, or the Daemons will be sneering condescendingly at their pawns...
So perhaps not the most effective army build, but incredibly fun and accurate to the guiding idea behind the list!