When an enemy engages you, or you optionally engage an enemy, it leaves the staging area and is considered "engaged with you". This engagement is between the player and the enemy, not a particular ally/hero and the enemy. You will find cards that talk about "enemies not engaged with you" in future expansions.
Additionally, there is no need to exhaust a character as part of engagement unless a card specifically states otherwise. If you are in the combat phase and have already exhausted to defend against an attack from the enemy, it is too late for A Light in the Dark to prevent the attack. Even if you send the enemy back to the staging area, your defender is still exhausted and you must reveal shadow cards, calculate damage from the attack as normal.