Ok, here's where my beef comes in. This last sentence has an 'and' in it. That means both effects are bound to each other. The second effect cannot resolve if the first one doesn't, IMHO. So, I believe the answer(s) to FelixChCh's question would be...
If I select my opponent's "
Horus the Black Flame Dragon LV6" and one of my monsters, the effect will
Disappear because "
Horus the Black Flame Dragon LV6" cannot be destroyed by the effect of "
Mystic Box". And since my opponent's monster wasn't destroyed, then control of my monster will not switch to my opponent.
If I select my own "
Horus the Black Flame Dragon LV6" and one of my opponent's monsters, then their monster will be destroyed, but I will keep my "
Horus the Black Flame Dragon LV6" because he's not affected by "
Mystic Box".
That seems kind of unfair don't you think?