Istvaan V was a dark, desolate world. A planet desecrated by black volcanic ash from eruptions long since passed, this world was practically worthless in terms of resources and habitability. But for the now-traitor Horus Lupercal, this was the perfect place to set up his current cadre of three other traitor Legions -- the World Eaters, Death Guard, the Emperor's Children on the Urgall Depression. He has spent considerable resources, time, and manpower in repairing and reinforcing the old fortress that was at the very top of the Urgall Depression, and beckoned the three loyalist Legions -- which consisted of the Salamanders, Iron Hands, Raven Guard, and their "reinforcements" to come down and crush them after the atrocity of Istvaan III. Your role here is also vague, and you shall decide what you're going to be.

Loyalist Primarchs (Idk if I'll do the traitors as well):
Vulkan:

Ferrus Manus dueling with Fulgrim:

Corvus Corax and Konrad Curze dueling (not on Istvaan 5, but both were present.):

Art credits go to: (Jeff Porter for Vulkan, Anastasya Zakharova for Corax and Curze, Jeff Porter for Ferrus Manus and Fulgrim.)