Originally Posted by MontytheMighty
To me, all the hints regarding "a primarch killing another primarch" seem to be referencing Ferrus Manus vs. Fulgrim.
When Mortarion confirms Ferrus's death, that confirmation alone is enough to wrongfoot the Khan, who is momentarily taken aback. I don't think the Khan would've been that shocked had another primarch already died by the hand of another primarch early in the Crusade.
But I think there's a bit where it's Yesugei thinking to himself about it before he comes across the Shattered Legion survivors. This is what led me to think there were rumours previous to the Heresy regarding it.
Personally I think it might have been Guilliman that did the deed instead of the more obvious choice of Russ, though.