Here's their dialogue:
Close now, he saw the strain upon his father's face, and realised what it had cost to hold the way open for him and his sons, though they had been unable to take it and had instead come via a different road, one that led to the earth itself.
And at last, with the talisman of seven hammers in his hand and the Throne within his reach, did Vulkan see.
And the horror of it, what his father had used him to create, the entire purpose for his resurrection, came crashing in.
He shut his eyes, the light burning, and when he opened them again he was no longer on Terra.
He had returned to Nocturne.
A man faced him, slighter of frame and wearing a strange garb that put Vulkan in mind of a Grekan Myrmidone of Old Earth. A long tan cloak swept across his right shoulder, pinned at his breast with a circular bronze stud. Around his waist was a thick belt of pteruges, and he wore a gold breastplate sculpted to resemble a man's naked musculature. He had no helm, instead preferring a silver circlet.
His dark hair flowed like a mane of jet. Torcs ringed his arms and he wore vambraces and shin guards in the same style as his breastplate.
Vulkan frowned. 'Why have you done this, father? I have fashioned something… abominable.'
The Outlander's gaze flicked to the talisman around His son's neck and then back to Vulkan.
'Do you recall what I said to you, as we sat here and looked out upon these very sands?'
The great expanse of the Pyre Desert stretched out before them, harsh and unforgiving but beautiful in its way.
Vulkan did not answer, and he did not meet his father's eye. Heat haze made the desert tremble, even with the sun setting and painting the sand a fiery red.
'I said your destiny was a great one,' the Outlander went on. 'And I said―'
'That you needed me more than I knew, more than perhaps I would ever know.'
Vulkan shook his head, a grimace of denial set upon his face. 'But this... How can I do this?'
'You are the earth, my son, its fire and solidity, that is how.'
'And the great flame that it will unleash if the Throne fails, if you fail?'
Vulkan felt a firm hand upon his shoulder and heard the darkening of his father's mood in His words.
'It will consume the Palace and all of Terra. The Throneworld will burn.'
A sharp turn brought Vulkan eye to eye with the Outlander. Incredulity warred with duty on his face.
'To deny it to my brother?'
'No, my son,' said the Emperor sadly, the Master of Mankind standing before him now. 'Not to deny it to Horus but to Chaos, and to strike a blow against their forces the like of which they will never recover from.'
'To win the war you would sacrifice Terra?'
'If Terra falls and Horus takes it then we have lost anyway, and all of mankind will suffer.'
Vulkan looked down at the talisman in his hand and fought the urge to crush it, though he knew it would not yield to even his strength.
'I am sorry, my son,' said the Emperor. 'I needed to hide it from you, what you had created and what would be wrought by it in my name.'
'Has this… Has it always been within me, the capacity to fashion such a thing?'
'Tell me, my son, why did you destroy your great works and send the others where no one might ever find them?'
'I feared they would be put to ill use, and what they could unleash.'
'Which is why it had to be you. It has ever been within you, Vulkan, and I hoped such a day would not come to pass that I had need of it… I hoped for a great many things,' He said sadly. 'Your death, your true death and resurrection, brought forth what you needed to fashion the talisman. All of your pain, the suffering of your Legion, it has led us to this point.'
Vulkan met his father's gaze, defiant.
'And if I refuse?'
'You will not refuse, for you still believe in hope that I will prevail, that Horus will be stopped and the war will end. But you are also pragmatic, and know that this must be done in case hope fails us in the end.'
And as before the light grew around the Emperor, radiating from His skin and Vulkan shut his eyes lest he was struck blind.