Theory on the missing Primarches from Angel Exterminatus

[Custodian Athiair]

Okay I know there is about a hundred of these each month, wiyh different people thinking about potential reasons as to why the two primarches/legions were wiped from record.

 

But having *almost finished Angel Exterminatus, there was something that stuck with me about two-thirds of the way through. It mentioned that each of the Primarches has to climb the Astartes Tower on Terra to swear their Oath of Allegiance to the Emperor for the Great Crusade, and Perturabo is thinking about his climb. And he mentions that it wasn't just a physical climb but they were also tried mentally as they climbed, and he also then mentions that all of them had taken the climb. But not all of them has succeeded in climbing it. 

 

Now my querey is, who "failed", why they failed, and if this was the final test for the Primarches and those that didn't complete weren't worthy to lead the Emperors armies???

 

I'm sure that there are some mentions that they did take part in the Great Crusade at some point, but this just pulled at my head a bit making me think about it.

 

Opinions?

[Mantras]

Already a thread on this exact topic : http://www.bolterandchainsword.com/topic/271603-missing-primarchs-a-simpler-than-expected-answer/

[Captain Semper]

Since the link is provided already this one gets locked.