I don't make a move because at this point I'm mystified and don't even know how to react. As I'm standing there, it looks like he clicks and drags a bunch of the hollows and drops them closer to me, since I'm not moving close to engage him. It's literally raining hollows all around me. I really wish I had recorded this happening somehow, it was pretty cool.
I messaged him afterwards and he said it's a mod, it only works on enemies that are present on the map. Just think of the possibilities: you're in the Depths when suddenly curse frogs everywhere!
I thought I'd share with everyone this odd thing that happened.