HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

I'm not entirely sure, but analyzing the code, I can say that only the selected (active) hero can disappear like that. If you leave the focus on the city, for example, this won't happen. And physically (if I can put it that way), it doesn't actually disappear; only the graphic representation disappears. I'll see what I can do...