At level 6th, Warlock Undead patron have features call "Grave Touch"

I want to know that which effect can cause additional damage die for determining necrotic damage?

So, the ability says:

In addition, once during each of your turns, when you hit a creature with an attack roll 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.

I'm not entirely sure what you're asking, but: When the character has their Form of Dread (1st level Undead warlock ability) active, they get to roll an extra damage die whenever they use this ability.

So, if they use it on one of the bolts from their Eldritch Blast, that one does 2d10 necrotic. 

If they throw a fireball (yes, I know they don't have fireball; I'm just too lazy to look at their spell list), they don't get to turn it necrotic, because it's not an attack roll they make, but an effect the target saves against.