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Do you think that it would be alright to use 40k Chaos Space Marine models in a Horus Heresy army? I was thinking along the lines of having a Death Guard Plague Marine army that would would be a portion of the forces that Typhus used while he was away from Mortarion. Not all of them would be Plague Marines, as I want to get some Mark IV Marines, but I would like to have at least some of the army be corrupted. I was also thinking about having a possessed Word Bearers army, and/or painting some of the Shadowspear CSM models like the Sons of Horus. Would this be an acceptable thing to do?

Short answer, yes but no.

Long answer, the rules for HH aren't at the point where the Death Guard has become corrupted by Nurgle. Currently, they are still just regular Astartes and the most you'd find Chaos wise is some little tokens that could be passed off as Lodge badges. I'm sure it will get to that point eventually when the Siege is reached in the rulebooks but right now even a mostly corrupted Death Guard army would get you some scowls from some HH players.
The general issue arises between the feel of the setting and "Your Dudes". For example, there is a fair chunk of hobbyists who use cleaned-up 40k Death Guard models for "true scale" Astartes as the proportions are larger and they emphasise the "monster encased in massive armour" aspect of the background. These might not be the same as every other HH army but they fit the setting. Some people have also used the Primaris Outrider Bikes (not the riders) as they are chunkier and more brutalist as well, again fitting the setting. However, Primaris conversions generally don't fly and people get a lot of flak (sometimes pushing into unreasonable hate) for doing them. "Your Dudes" are by definition yours and really nobody should be getting on your case for using models you like but HH has a strong "it's a historical game" vibe that means people who don't follow the crowd either get lucky and get praised as artistic geniuses or get lynched as trying to destroy the setting and game.
As the story has moved closer to the Siege of Terra, hobbyists have been getting to see the Traitor Legions move onto the more Chaos-heavy versions, especially the Death Guard, Emperor's Children, World Eaters, and to a degree the Sons of Horus and Word Bearers. Within the Siege stories and on the covers we're seeing things like these:


We have explicitly Chaos Astartes now in the story but the rule books haven't caught up to that point so there is an impasse between those who want to play the story as it is and those who haven't moved past Thramas because that's where the FW series ended (sort of).

Thanks alot for the information. I think that I'll hold off on doing 30k Plague Marines for now. In terms of the Word Bearers, they were the only Legion who were into Chaos worship right off the bat. As I'm big into CSM with mutations and such, would it be fluffy to convert 30k Word Bearers to have stuff like claws and sharp teeth? Their whole modus operandi was Chaos worship, with everything else being secondary for them, to the extent that Horus kicked them off his flagship. Finally, what about converting greater possessed into Gal Vorbak? They have the same base size and the only major change that I would have to implement would be to give them bolters. The FW models are great, but are of course way expensive.

I'm generally fully supportive of 40k CSM as late-Heresy Traitors from any Legion, particularly if your force went off the deep end with the daemons fast.

At best for the Word Bearers I'd maybe use the odd CSM weapon rather than using them consistently. Chaos worship was their thing but that didn't really translate to mass mutations until after the Heresy. And to be fair it was just Erebus and Lorgar that got chucked out, the former because he was a worm and the latter because Lorgar tried to overthrow Horus.
As for the Gal Vorbak, they come in units of 5 and the Greater Possessed aren't a brilliant kit if you want more than 2. There are rumours of new Possessed coming so maybe wait that one out.