The Undead warlock has a feature called Grave Touched that states:
Grave Touched: At 6th level, your patron’s powers have a profound effect on your body and magic. You don’t need to eat, drink, or breathe. In addition, once during each of your turns, when you hit a creature with an attack and roll damage against the creature, you can replace the damage type with necrotic damage. While you are using your Form of Dread, you can roll one additional damage die when determining the necrotic damage the target takes.
The part I am interested in is the part that states that you can roll an extra damage die when determining the necrotic damage a target takes. Would it be possible to use this feature on any source of necrotic damage, or does it only apply to attacks converted to necrotic damage with Grave Touched? I'm thinking of making a Topaz Dragonborn warlock and am wondering if I could use Grave Touched to amplify the necrotic damage from the Topaz dragonborn's breath weapon.