Is the damage from the Undead warlock's Grave Touched feature considered weapon damage for the purposes of Great Weapon Fighting or similar features?

Question

The Undead-patron warlock's Grave Touched feature (VRGtR, p. 30) says that when you hit a creature with an attack, you can replace the damage type with necrotic damage; it also adds 1 more damage die while you're transformed by your Form of Dread feature.

Is this damage considered "damage for a weapon" for features like the Savage Attacker feat or the Great Weapon Fighting fighting style?
Or would using Grave Touched actually make it not weapon damage, and therefore even the base die is no longer eligible for Great Weapon Fighting or Savage Attacker?

Answer 1

The weapon damage dice that have their damage type changed can be rerolled

The intent of the Great Weapon Fighting Style is spelled out in the Sage Advice Compendium:

Q. If you use Great Weapon Fighting with a feature like Divine Smite or a spell like hex, do you get to reroll any 1 or 2 you roll for the extra damage?

A. The Great Weapon Fighting feature—which is shared by fighters and paladins—is meant to benefit only the damage roll of the weapon used with the feature. For example, if you use a greatsword with the feature, you can reroll any 1 or 2 you roll on the weapon’s 2d6. If you’re a paladin and use Divine Smite with the greatsword, Great Weapon Fighting doesn’t let you reroll a 1 or 2 that you roll for the damage of Divine Smite.

It only impacts the damage roll of the weapon, so only the weapon damage dice themselves. Grave Touched does not change that the damage dice come from the weapon itself; it exclusively changes the damage type. So any weapon damage dice can still be rerolled with Great Weapon Fighting.

---

The intent is that the additional damage die cannot be rerolled

When a feature adds weapon damage dice it states so. Let's compare the Cavalier Fighter's Unwavering Mask feature, the Half-Orc's Savage Attacks trait, and the Barbarian's Brutal Critical feature:

[...] the attack's weapon deals extra damage to the target equal to half your fighter level. [...]

[...] you can roll one of the weapon’s damage dice one additional time [...]

[...] you can roll one additional weapon damage die [...]

All of these features specifically mention that the weapon itself gains additional damage or that you specifically roll additional weapon damage dice. Weapon damage dice are precisely what Great Weapon Fighting allows you to reroll but the Grave Touched feature does not add weapon damage dice. Instead it states:

[...] you can roll one additional damage die [...]

Because this additional die is not a weapon damage die, it cannot be rerolled with Great Weapon Fighting.

---

The rules as they are written, do let the additional damage die be rerolled

That entire last section relies on trusting the Sage Advice Compendium and that SAC answer even states that Great Weapon Fighting is only meant to benefit the damage roll of the weapon. If we look at the actual wording of the feature, the text does not support this stance:

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die [...]

This simply activates when you roll a damage die, not a weapon damage die. So following the rules as they are written, we actually could reroll Divine Smite, hex, Sneak Attack, or even Grave Touched. However, we know that this is not the intent of the designers. Do with this knowledge what you will.

Answer 2

Yes, the affected weapon damage dice can be rerolled.

The description for Grave Touched states the following:

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.

As a basis, let's consider the general case where you replace the damage type with necrotic: the only thing this is doing is replacing the damage type--the rule says nothing about reclassifying the source or "type" of the damage. Nothing beyond this changes, so there's nothing to indicate that this would modify interactions with features like Great Weapon Fighting, which say nothing about damage type.

What about the more specific case where Form of Dread allows you to roll an additional damage die? In this case, it still doesn't matter, because this is not bonus damage (which Sage Advice states is not affected by GWF) but an additional damage die. This is important because it functions as if the weapon in question had an additional damage die, allowing it to be affected by GWF or Savage Attacker. This is no different from how critical strikes interact with GWF.

As such, you can reroll these die using both Savage Attacker and Great Weapon Fighting.