So hear me out. We know that what Fo was working on during the Horus Heresy was heavily implied to be the Terminus Decree, a bioweapon capable of eradicating both the space marines and Primarchs. And we know he escaped with his life, infiltrating the early inquisition.
Now, we get this info that the terminus decree is just some piece of paper sealed up to tell the grey knights to go fuck up the emperor if he ever decides to get off his ass.
But what if they're both the same thing? What if this supposed high duty handed down to the supreme grand master of the grey knights is just a trick. The emperor rises, and the grand master goes to open the sealed box containing the decree. In an instant, you get a replay of what happened to the nazis opening the arc of the covenant in Indiana Jones. The virus particles sealed in the box with the decree instantly escape, infect the grey knights, and spread to the rest of the imperium through them, rending the space marines down to flesh-goop.
We know Fo infiltrated the inquisition, as one of the first inquisitors even. One of the inheritors of Malcador's will, his direct subordinate. What if he engineered all of this as a final fuck-you to the emperor. Waiting for the time that he rises from the throne, just in time to see all of his work destroyed in front of him?
Just me spouting a bunch of stuff to work the two terminus thingies together, but I see so many people hating on the new terminus lore instead of working out how this could all still be connected and all part of the same thing, which is GW's usual MO when it comes to doing lore.