Sister,’ said the false sibling. She reached out and touched the shivering Clara. ‘For I may not be your sister in blood, but we are Sisters in duty. These tyranids are unfamiliar to me. My service was spent fighting the Heretic Astartes. But I know my duty – to protect my lord’s life, and defeat all enemies of the God-Emperor. I know not whether my lord lives or has fallen to the heretics, but I can see that the enemies of the Imperium are outside that door.’ She nodded at the plasteel, then turned to Trazyn. ‘You, xenos. If we do this service to you, will we be free to make our own destiny?’
‘I will not return you to this exhibit,’ said Trazyn. ‘I swear on my honour. I can reunite you with your lord, if you wish it.’
‘I wish it,’ said the Sister, with a nod. ‘My name is Magdelena, by the by. And now, Sister…’ She grasped Clara by the shoulders and looked into her eyes. Bright fires glowed within, the light of holy faith. A righteous certainty that stilled the nerves of her rattled companion. ‘Let us kill this alien, Sister. For the Emperor–’
‘For the Emperor,’ echoed Clara.
‘…and Lord Vandire,’ Magdelena finished.
...
And he had outdone himself on the poses. Clara stood before it, the spent shells of her storm bolter frozen as they arced through the air. The magos, conscious this time as requested, leaned over the controls of his gauss flayer turrets, two spike rifle rounds in his back. Trazyn would remove them when the magos was needed again.
His own double, a surrogate, stood at the control cartouche. And Magdelena covered the back entrance with her meltagun. That back entrance hadn’t existed in the real Nexus Mundi, but he wanted to position her so that she could see the tableau directly across: The Beheading of Goge Vandire.
He had, after all, promised that he would not return them to their exhibit – and this new feature, titled War in the Museum, was indeed not their original exhibit. So they stood with updated identification plaques, preserved for eternity.

War in the Museum