Board Thread:Warhammer 40k General Discussion/@comment-6078851-20130130174042/@comment-24944355-20140701093719

Yes Dorn was killed by a demon during the 1st black crusade - but no ordinary demon kills a primarch so I believe the demon on the ship was Perturabo as demon prince and that the whole thing was a trap Perturabo set up to get Dorn the one person he hated most in the world.

As for the Khan in scars it's shown that he has a prophetic dream of fighting a huge figure of darkness under a black sky and being killed by him. As the Khan's foe in the book was Mortarion and in Vengeful Spirit Mortarion swore vengeance on the Khan we can extrapolate that the dark figure who kills the Khan in the prophetic dream is Mortarion as demon prince who likely ambushed the Khan after he entered the webway.

True the Dorn hypothesis is pure theory but it has a lot of strong probability with it. While the Khan Hypothesis has a fair bit of detail behind it from the most recent heresy novels, you need to keep up with your reading.