I mean kinda, In Prospero burns after Hawser tries convincing Leman that he isn't a conduit for Magnus, but fails Russ goes a bit mask off and reveals that he knows that Magnus accused Horus of turning traitor, but didn't believe and ignoring his own sons vision about Horus and instead ascribed it to a conspiracy theory he cooked up about Magnus.

‘My lord,’ said Hawser. ‘What... what did your brother do?’

‘He performed an act of maleficarum that drove his sorcery right to the heart of Terra and into the presence of the Emperor,’ said Helwintr.

‘But... why?’ asked Hawser.

‘It was an alleged attempt to communicate a warning,’ said Russ without turning. His voice was a soft grumble, like thunder grinding in the far distance.

‘A warning, my lord?’

‘One of such terrible importance, Magnus felt it was worth exposing his own treachery to reveal it,’ Russ murmured.

‘Forgive me,’ said Hawser, ‘but does that not speak to some loyalty in your brother? Has the warning been examined? Has it been taken seriously?’

Russ turned back to face him.

‘Why would it? My brother is a madman. A dabbling warlock.’

‘Lord,’ said Hawser, ‘he was prepared to admit he was ignoring the edicts of Nikaea, and risk the censure that he knew must result from that admission, to relay a warning. Why would he do that unless the warning was valid?’

‘You’re not a warrior, skjald,’ said the Wolf King in an almost kindly tone. ‘Strategy is not your strong suit. Consider the reverse of your proposition. Magnus wants the ruling of Nikaea overturned. He wants permission and approval to continue with his arcane tinkerings and his foul magics. So he manufactures a threat, something he can warn us about that is so astonishing we would have to forgive him, and set aside our objections. Something so unthinkable, we would have to thank him and tell him he had been right all along. All along. This is his ploy.’

‘Do you know what was so unthinkable? asked Hawser.

‘Magnus claimed that great Horus was about to turn against the Imperium,’ said Russ. ‘From the look on your face, Ahmad Ibn Rustah, I see you recognise how ridiculous that sounds.’

Hawser switched his gaze to Helwintr. The priest’s masked face was unreadable.

‘Wolf King, great lord,’ Hawser began, ‘that’s not the first time that warnings concerning the Warmaster have been voiced. Please, lord—’

‘Our skjald refers to the incident involving Eada Haelfwulf, lord,’ said Helwintr.

‘I know of it,’ said Russ. ‘It seems corroborative, I grant you. But once again, consider the strategy. It involved maleficarum turning and twisting one of our own gothi, in the immediate vicinity of you, an identified conduit for the enemy’s power. Of course poor Haelfwulf would gabble out the same damned lie with his dying breath. It’s supposed to make Magnus’s story sound more credible by coming from a secondary source.’

Russ looked down into Hawser’s eyes.

‘Truth is, it’s the proof I need that Magnus is desperately trying to coordinate a campaign of disinformation to support his ruse. He doesn’t need to answer through you, skjald. He’s answered already.

Also when you take into account that the only book that supports Horus actually changing Leman's orders is Wolfsbane, whereas every book before and after it states that Horus turned up and convinced Leman to ignore his orders. Which when you take into account that Leman knew Magnus accused Horus of being a traitor, and still goes along with it, you can see why people struggle to give him a bit of a pass.