I'm currently writing a bit of fiction for 40k. It's about a custom chapter who are loyalist Sons of Horus. What I currently have right now is that a Black Legion Warband wants to lure the chapter to Istvaan III. The founder of the chapter (a survivor of Istvaan III) killed a greater demon there (He had help from like 50 other loyalists). So the Chaos marines manage to lure the chapter to Istvaan III. I have two ideas for what will happen next.
1. The Chaos forces attempt to sacrifice 100 of the chapter's Space Marines in order to resurrect Horus. Using essence that the demon which was slain by the founder of the chapter left behind as a focal point for the ritual. Objective: Resurrect Horus (or some malformed version of him) as a daemon-prince of Khorne.
2. The Chaos forces find a withered being, a human, on Istvaan III. This is the loyalist bit of Horus's soul which died/was separated from the rest of Horus (in the story) when he ordered the guns to fire the virus bombs. The Chaos forces want to corrupt this sliver of Horus's soul but need to have enough of the primarch's genetic data to make it physically stronger. Objective: corrupt the remnant of Horus's soul and transform it into a daemon prince of Khorne.
The concepts are a bit rough but I hope you get a general idea from this basic outline. Does anyone see any glaring problems with this? Is there one idea that you think is better than the other?