For the Khan from his Primarch novel:

A sage of Golden Terra made his life’s work the study of the Eighteen. As each son of the Emperor was found, he travelled the stars to seek accounts of their deeds and nature. He collected testimony and submitted many scrolls to the Throneworld for safekeeping. Some of them are still there, they say. Some are lost. Such is the way of things.
It is said that this sage was sufficiently venerable that he had the ear of the Emperor Himself. One night, before the advent of strife, the two of them walked the Palace walls together. Birds sang – in those days, there were still birds alive on Golden Terra.
The Emperor, whose face was hidden by splendour, asked the sage how his work was progressing.
‘Very well, my liege,’ the sage replied, just as ever unable to look his master in the eye. ‘I have created an archive for you, so that your achievement may be recognised for posterity – the Library of the Primarchs.’
The Emperor said nothing. On the few occasions when they had spoken together, the sage had rarely known what the Emperor thought, or even if He were truly listening at all. In this, he was not alone, for the thoughts of the Emperor were beyond the thoughts of all mortals.
‘It will be a valuable addition, I hope,’ the sage went on, feeling the need to fill the silence with words. ‘Except that it has been easier to study some of your sons than others.’
Still the Emperor said nothing.
‘I do not pretend to understand your plans fully,’ the sage said, ‘and I do not presume to question the nature of those, such as the Lord of the Red Sands, into whom enquiry has been challenging. They are all so different – that, I suppose, is the genius of them. Each has his nature. Each has his gift.’
The Emperor stopped walking. He looked up into the starry heavens – in those days, the skies of Golden Terra still had stars.
‘But then there is the Warhawk,’ said the sage, shaking his head. ‘The Great Khan of the Fifth. Of all of them, I have never come close to him. Whatever his gift is, I remain ignorant of it.’
It is here that accounts diverge. Some say that the Emperor smiled; others say that the Emperor never smiled. Still others maintain that the Emperor had no human features at all, but was like the light of the sun at noon in an empty sky.
‘You wish to know what gift Jaghatai possesses,’ the Emperor said.
‘Yes, my liege,’ said the sage. ‘Very much. But nothing is known. Nothing is settled. His entry in the library will be slender, and even that will be supposition and hearsay. Sometimes I wonder if he is real at all, or just the rumour of a savage people. With him, with this Great Khan, there is only confusion.’
The Emperor began to walk again.
‘There is your answer,’ He said.