You don’t have to see the horn to witness the effects.
Boring television/writing: “Here’s this awesome horn which will mind control everyone and get them to do what I say when you, Person I Have No Use For or Maybe Need Out of my Way, blow it for me and proceed to fall over dead afterwards” followed by a long, knowing gaze into the camera.
Watch the scene again, and pay attention to it. :)
Dude, I watched the scene. You can’t wish cast a major piece of equipment into a show that has one season left, which they are dividing into two.