If an opponent attacks a player or planeswalker but each creature attacking that player or planeswalker is no longer attacking that player or planeswalker by the time Tahngarth's ability resolves (most likely because they've left the battlefield), that player or planeswalker can't be chosen for Tahngarth to attack. If there's no player or planeswalker to choose this way but you still choose to give the opponent control of Tahngarth, Tahngarth isn't attacking.