If your “Legacy Hunter” attacks your monster (controlled by your opponent) on your opponent’s side of the field, “Legacy Hunter’”s effect still activates and your opponent returns a card to the Deck.
If “Legacy Hunter” attacks a face-down “Man-Eater Bug” or “Cyber Jar”, “Legacy Hunter”’s effect is not applied because “Legacy Hunter”’s effect activates when the monster he destroyed is sent to the Graveyard, and “Legacy Hunter” will no longer be on the field in these instances.
The monster that “Legacy Hunter” attacks must be face-down at the start of the Damage Step for “Legacy Hunter’s” effect to activate. So if “Legacy Hunter” attacks a face-down Defense Position monster and your opponent activates “Ceasefire” to flip the monster face-up, “Legacy Hunter’s” effect is NOT applied. Also, if “Legacy Hunter” attacks a face-up monster and “Book of Moon” flips it face-down, “Legacy Hunter’s” effect DOES apply.