[@Roosan] I will be controlling the Emperor, yes, and most likely a number of other NPCs where relevant. That said, you can make your Primarch in whatever way you like. They can be a psyker, a blank, maybe even a mutant, whatever works. Your characters don't explicitly need to be parallels to those in canon. Similarly, the story will develop based on player actions - as [@agentmanatee] says, it's up to you guys whether a betrayal/Heresy-esque event occurs at any point.