[quote=@BCTheEntity]
[@ClocktowerEchos] Now, regarding the whole fate thing... in 40K canon, Malcador the Sigillite sacrificed himself upon the Golden Throne to keep the Astronomican functional whilst the Emperor fought Horus. But of course, the only reason that was necessary is because Magnus broke the wall keeping all the daemons out of the Emperor's pseudo-Webway, and so far, Oph appears to be the only psyker Primarch... that said, if Ophiuchi manages to break the wall too, I'm wondering if he might wind up sacrificing himself on the Throne instead as a means of redeeming himself, or at least making the offer so Malcador doesn't have to. Then again, I'm not sure if he'd even be powerful enough to do so, since Magnus needed a boost from Tzeentch to do so in the first place, and he was also using Sorcery that Ophiuchi wouldn't even consider as a valid option. So maybe not, after all... I ought to think about that.
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As of right now, I think the coGMs and I have decided that the fate is what you "expect" to happen (in loose terms). This means that it will be roughly the path you'll go down but if there's some event that you think should alter their fate then you can do it. 

IE: You write that Ophi ends up on the throne but somehow finds a way to allow for the astronomicon to work without someone sitting in it and just feeding it more psykers or Talvyrn, instead of dying in the siege of Terra, ends up surviving and decides to go on a one man crusade with some of his homies straight into the eye of terror to dickpunch some daemons.

Basically its the road you're going to be taking under the assumption nothing else will peak your interest otherwise but you are more than free to take a turn on to a different road if you see a plot point that interests you.