Here's why:
* Fiendish Chain's activation condition: "When an opponent's monster declares an attack: You can banish this card from your Graveyard; negate the attack and destroy that monster."
* Declaration of attack: The moment your opponent declares an attack is the trigger point for Fiendish Chain's activation.
Therefore, you can activate Fiendish Chain during the Battle Phase, specifically when your opponent declares an attack.
Important Note: You can only activate Fiendish Chain from your Graveyard, not your hand or field.