His pupils constricted. It was an involuntary physiological contraction, impossible to fake—the black circles at the center of his irises narrowed in that instant, the muscle fibers at their edges switching from relaxation to contraction in an exceedingly brief span of time.
Normally, such a contraction appeared only when someone faced mortal danger. Yet there was no harsh light here, nor any visible threat. There was only an old acquaintance standing two meters away, quietly watching him after speaking a hidden truth aloud.
At the same time, Dumbledore's presence underwent a silent expansion in that instant. No offensive magic spilled forth, no sparks or beams of light appeared, yet the force of that expansion covered a spherical radius of roughly ten meters around him—the blades of grass within it bent away from him all at once, fine concentric ripples spread across the amber surface of the fountain pool, and three bees resting at the edge of the flowerbeds took flight together, as though startled by a low-frequency sound wave humans could not perceive but living creatures could sense.
The expansion subsided in less than a second. Dumbledore drew that power back into himself, like a fist extended through the air curling shut and withdrawing into its sleeve. His pupils returned to normal as well.
Yet not a single person in the garden stepped back. The Portuguese wizard even raised his wineglass and took another sip, unhurried and composed. A Middle Eastern potions merchant in a headscarf rested one arm over the back of his chair and shifted into a more comfortable position. Then he looked up and gave Dumbledore a faint smile, as if he had just finished the final detail of a painting.
Slughorn did not move a single step. He even tilted his head slightly, at an angle