Board Thread:Warhammer 40k General Discussion/@comment-29025531-20170413061825/@comment-24944355-20170414002440

Of the missing primarchs Dorn & the Khan are the most likely to be dead. Dorn fell when he boarded a Chaos warship during the 1st Black Crusade opnly to come face to face with a demon prince. As no ordinary demon could take down a primarch & given the enmity between the two, my money is on the Demon being Perturabo  if that was the case i think it unlikely that Dorn survived the battle. Still GW left just enough wiggle room that if they really wanted to they could bring him back.

As for the Khan he had a vision of his death battling a giant figure of darkness on a dark plane under a starless sky. From the hints given it probably means he fell to Mortarion after entering the webway though like Dorn GW left just enough wiggle room in that either the Khan was captured rather than killed or the dual is still to come so as to leave an out if GW decides to bring him back.