Iron Within (Warhammer+ series)
Iron Within is another fan-animation that was bought by Games Workshop at least two years ago (probably somewhere in 2021), it was originally named Codex and was finally released on their polarizing Warhammer+ but as expected of greedy GeeDubs, they couldn't help but somehow botch it. The visuals are top notch, but the script, editing and sounds are pretty much what would you expect when Games Workshop decides to get their grubby mitts on something that was supposed to be Awesome. You can check out the trailer here.
Overview
It's a 27-minute one-shot animation that focuses on an beleaguered Imperial Guard planet that's being raided by a sadistic Dark Eldar raiding party. Whisking away it's citizens to Commorragh and reaping a large death toll on the planet's defenders. Behind the barricades, the Imperial Guard are pushed to the point that they turn to an ancestral emergency beacon to call upon the Emperor's Angels of Death, but if you actually bothered to read the title of this animation then you have a very good idea on who answers this ancestral lifeline instead...
Imperial Guard not recognizing Chaos Space Marines?
It's more likely than you think.
When the planet's Imperial Guard regiment (or more likely their local PDF) end up being saved by the Iron Warriors, they don't seem incredibly alarmed and in fact confuse them for your usual Loyalist Astartes. This might seem dumb at first but there's a very good reason for this, remember the scale of the 40k universe is fucking HUGE, and while Adeptus Astartes are the poster boys they aren't exactly everywhere in the galaxy. Normal people in the 40k universe are more likely to see the Astra Militarum in action rather than a one of the Emperor's Angels of Death.
To further drive home that the average Imperial citizen has no idea what a space marine looks like. The early human settlers on the planet of Serenade, confused a bunch of Necrons (lead by Trazyn and Orikan) that protected their planet (which was secretly a Tomb World) from an Ork invasion for Space Marines and actually erected statues in their honor across Serenade.
This also allows us to have a darkly hilarious scene of a Guardsman doing the sign of the aquilla right in front of an Iron Warrior whose response is to just smack him on the face and walk away. This display of petty dickery really makes you wonder if the Marines Malevolent actually are a Traitor Legion Loyalist of the Iron Warriors.