Grave Touched 6th level Undead Warlock Feature, my question concerns the underlined:

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

So is this one extra damage die every time the target takes necrotic damage or is it one extra die on the hit that you replaced only, which can only be done once a turn.

A few examples at 6th level:

1. Eldritch Blast in Form of Dread

I am using Eldritch Blast and Hex while in form of dread - Normal Damage is 1d10 Force + 1d6 Necrotic for each blast.

So now I use Grave touched, so I switch out the first one and it does 1d10 Necrotic+1d6 Necrotic ...  I add an extra 1d10 to make it 2d10+1d6 Necrotic

Then I hit with the second blast .... it does 1d10 Force +1d6 Necrotic.  Can I add another 1d6 necrotic for 1d10 Force + 2d6 necrotic? 

2.   Arms of Hadar (3rd level cast)

So this does 3d6 Necrotic to everyone within 10 feet.  It is not an attack so I can't replace a die.   However it does do necrotic damage, so can I add  just1d6 for 4d6 total using the second part of the paragraph?

It's a bit sloppily worded, but I suspect that the intention is that the underlined part of this feature is supposed to be on top of the first sentence, so it would still be limited by the same "once during each of your turns" condition, and need to be part of an attack.

If it were intended to be its own entirely separate thing I would have expected it to start with something like "In addition, whenever you deal necrotic damage while using your form of dread" or similar, but they've definitely left it wide open for interpretation, so it's going to be up to your DM.

Grave touch clarification.

I have fighter as a multiclass. If i make a second attack with extra attack in form of dread, does grave touch add to the second attack or just the first attack.

The confusion is on if while in form of dread the additional damage die is “once during each of your turns” or just the changing to necrotic. 

Short sword 1d6. Form of dread active. 2 attacks made. I understand the first attack but on the second attack is it:

A) 1d6 Slashing + 1d6 Necrotic

B) 1d6 Slashing

First attack 1d6 (changed to necrotic) 1d6 (with form of dread addition)

Second attack 1d6 slashing - (Then is it additional 1d6 thanks to form of dread?)

Quote from kiyoshikengames >>

Grave touch clarification.

I have fighter as a multiclass. If i make a second attack with extra attack in form of dread, does grave touch add to the second attack or just the first attack.

The confusion is on if while in form of dread the additional damage die is “once during each of your turns” or just the changing to necrotic. 

Short sword 1d6. Form of dread active. 2 attacks made. I understand the first attack but on the second attack is it:

A) 1d6 Slashing + 1d6 Necrotic

B) 1d6 Slashing

First attack 1d6 (changed to necrotic) 1d6 (with form of dread addition)

Second attack 1d6 slashing - (Then is it additional 1d6 thanks to form of dread?)

No it is only once a turn.

Keep in mind your attacks happen in sequence, so there is both a hit roll and a choice involved and what happens on those things with the first attack affects what happens on the second.

So you make your first attack and there are three possibilities:

1. You miss - 0 damage

2. You hit and choose not to replace slashing with necrotic - 1d6 slashing damage

3. You hit and choose replace slashing with Necrotic - 2d6 necrotic damage

Then after your first attack, you make a second attack.  This is conditional on what happened on the first attack.  If #1 or #2 happened there are the same 3 possibilities.  If #3 happened on the first attack then only #1 or #2 are possible.

Makes sense. Thank you :)