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.