How does Undead warlock 'Grave Touched' interact with aoe [duplicate]

Question

I am confused about how the undead warlock's Grave Touched ability interacts with spells like Fireball. The text for Grave Touched reads as follows

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.

Emphasis on the "a" meaning it affects a singular creature. Does that mean when you cast Fireball and hit several creatures then would only one creature take necrotic damage while the rest take fire damage? Finally how would the extra damage die work with an aoe spell like Fire ball? Would every creature hit by it take the extra 1d6 damage or would just one creature take it?

Answer 1

Area of Effect spells aren't attacks - anything that cares about "an attack" will only interact with things that have you make an attack roll to hit a target.

If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.

(phb 194)