Just your proximity. Try to stay within... say, twenty feet.

[ It'll work a bit beyond that range, but just in case.

He flips the case open and plucks a tiny vial of ash from within, then pops the cap off the vial and dumps it on the ground with a murmured incantation. There's a sensation like a blanket settling over the two of them, and while the ambient sounds of the city and fighting remain crisp and clear, all sounds made by Akechi and Edelgard become more subdued and muffled. Even when Akechi talks it sounds like he's on the other side of a thin door. ]

I can maintain this for about an hour. After that, we'll need to sneak around the hard way.