How many damage dice does an Undead Warlock get to roll with Eldritch Blast?

Question

In dnd 5e the Undead Warlock has the Grave Touched ability starting at 6th level. Most of the ability is for role playing but the ability says

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.

If the warlock is using the Eldritch Blast cantrip does that mean they get to roll another d10 of damage if they choose necrotic instead of force damage? This would be the equivalent of getting yet another beam on a spell that already gets spammed because it is so powerful.

Answer 1

Once during each of your turns…you can roll an additional damage die.

We can pick the answer out of the quote you provide:

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.

So yes, it is similar to one additional beam of eldritch blast per turn, but only applies while you are using your Form of Dread feature. A warlock using this gets the damage die equivalent of another beam, but they do not get to apply the additional damage of Agonizing Blast to it. So a 6th level warlock with Agonizing Blast would deal the usual 1d10+CHA per beam and an additional 1d10 to one of the beams.