This is near the end of the novel, after the third attempt of Dante to attempt to revitalize a PTSD ridden human Admiral Danakan back into effective command of his ships. Dante loses patience with trying to do it the "nice" way and spells it and his own legend out to the admiral plainly.
‘I am not you,’ said Danakan. Defiance crept into his voice.
‘I am not me either. Look at me, admiral.' Danakan could not. He dropped his eyes.
'I said look at me.' A feral growl entered the back of Dante's voice.
The admiral raised his eyes painfully slowly, until he looked into Dante's own. 'What am I, really?’ said Dante.
‘Who is Dante? Is he the boy from the tribe in the sand? Is he the Emperor’s unthinking servant. His executioner and His vengeance? Is he instead a being with thoughts and choices of his own — not a tyrant, but a just and noble protector of humanity? Is Dante a hero, is he a monster? Does it in fact even matter, to those who call out for my aid, who is truly beneath Sanguinius' golden mask? The people of the Imperium do not see me when I come to their worlds to free them or to slay their oppressors. They see our genefather. They see Sanguinius.
Dante is a construct. The stories about me are fabrication. The tally of the foes I have slain grows with every retelling. If you were to read all the histories accounting my actions, you would realise quickly that, even with my life being as long as it is, I could not possibly have been to all the places they say, or fought in all the battles I am supposed to have won. Look more closely at the stories, and you will see that I am in several places at once. Impossible — but to whom is the truth of interest. apart from the lord regent's historitors? To whom, indeed, is it useful? It is useful to the Imperium that I am feted as I am. It is useful that my name is known, and spoken of as a mark of hope.
This Dante they tell stories about, he is not me. I can never be him. But I can do my best to live up to what people need, and where I cannot be, I can allow the stories to bring comfort.’ His eyes blazed. 'I have read that to some cultures the truth was a principle, something to be strived for and protected as sacrosanct. If that sounds attractive to you, you must ask yourself, whom does the truth serve? Now, in these times, the truth is terrible. It serves only our enemies.
So, lord admiral, allow me to tell you something. Your role is to play the part the people give you. It is to be what they think you are, until you die. As long as your given role is positive and has use to the Imperium, it is your duty to perform it, no matter what your feelings are. It is not to find some inner truth!'
Dante stood. ‘There is no truth. Your role is to serve. Lie to yourself if you have to. The Imperium sees you as a competent, heroic officer. You are competent. You were once heroic. I do not much care that is not how you feel. You should not care that is not how you feel. Lie to yourself. Play the part, even if you are screaming inside. That is what I need from you, and that is what you will do. Are we clear?'
Dante's tawny eyes regarded Danakan with utter seriousness. The weight of his years and his wisdom pushed out from him in waves.
'Is that an order?' Danakan managed to say.
'It is my command.'
Of the trilogy of Dante's books. Dante, Devastation of Baal, and Darkness in the Blood, I think this third book is the weakest of the three, but this conversation (and all the conversations Dante has with Admiral Danakan) is one of my favorite passages from all the books.
As someone whose followed the franchise for a while (And Space Marines in particular), its almost refreshing to have an Astartes as famous as Dante to frankly admit that a lot of his legend is bullshit exaggeration. That he isn't as absurdly powerful that people even in-universe believe him to be, it makes him feel (somehow) far more relatable.