Geron has the servant turn so the mirror he's admiring himself in allows him to see the Techpriest in the reflection, a magnanimous action and one he expects to be rewarded. Or else.

"Fine, tell your story. Make it a good one and I'll probably leave you alive."

The shriveled husk bolted to metal doesn't seem to have much good pain left in him, but he might as well gift the man to a lesser servant of Slaanesh, see if they can torture him into insanity. Or sanity given his reaction to the impending death of his planet.

What a wretched place, they were doing the universe a favor by obliterating it. Geron wondered if he should order some of the Technpriest corpses gathered and drained of blood, he'd had Technpriest blood before but maybe this would be a different flavor.