Horus and the Emperor are reality warpers now

Yet,' says Sanguinius, 'I suspect you are a distraction designed to delay me, so–'
'I am,' says Ferrus. His silver eyes are hard. His mouth moves out of joint with his words. 'All of this is. A display of power.'
'As I thought–'
'No, Sanguinius. No. Not as you thought. That's what I'm trying to tell you. Warn you, I suppose. You have no idea of his power.'
'Lupercal?'
'Yes, Lupercal. The power of his will alone is letting me be here.'
The shadows shift and rustle again.
'But I'm not a trick,' says Ferrus. 'I'm not an illusion, or some deceit conjured up from the immateria to divert you. You know that, don't you? I can see you do. I'm dead, Sanguinius, but I'm here. I'm real, and I'm me, and I'm dead, and I'm here. That's how powerful he is. He doesn't need to make a ghost of me, or magic up some vision that looks like me. The warp is in him to such a degree, he can simply bring me here from the other side of mortality.'
'To fight me? To stop me?'
'Oh, brother, no. To impress you. To show off.'
'Then I'm impressed,' says Sanguinius, 'but I'm still going to kill him.'
A pained smile slowly cracks across Ferrus Manus' face. It's a wounded version of a smile Sanguinius has long missed, and it tugs at his heart.
'And I'm going to watch you do it,' says Ferrus, his eyes bright, though his mouth is no longer moving with his words at all.
He clasps his gleaming hands together. He stares at Sanguinius intently.
'That's the thing about the warp,' he says. 'And I don't think our first-found brother has realised it yet. He's too drunk on its power. He can do anything now. Anything his will desires. You can't imagine. He can melt the world into the sky. He can blow time away like the tufts of a dandelion. He can twist up all of the materia in our universe into one knotted ball, and summon inevitable cities. He can drag the dead from their tombs and their times, and make them live as they once lived. But there is no finesse. It's the sport of a child.'
'You're saying he has no control?'
'He has plenty. But the warp is immeasurable. He's split the empyric realm open, and while he plays with one thing, or delights in another, or masters a new technique, it is wildly spilling out around him, following its own impulses. He put me here on a whim to greet you. I don't know what he was thinking. To surprise you? To remind you that death is always near? Maybe he thought the sight of your first-lost brother would chasten you or drive you mad. Who knows? Maybe he thought I would tempt you.'

-Siege of Terra, The End And The Death: Volume Two