TwoSix
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Thinking about it a little further, even if I did narrate it as a fireball+ to emphasize the death knight was also a competent mage, I would have some kind of rationale as to why it was different. Something like "Three times per day, as a free action, the death knight can infuse an energetic spell (one that does fire, cold, acid, or lightning damage) with vitality draining necrotic energy. The spell that is being cast does an equivalent amount of dice as necrotic damage.)I think that's why they don't just call it a "fireball" anymore. Because it isn't in a D&D sense.
It does 10d6 because the death knight is casting it as a 5th level spell equivalent.
I freeform that kind of narration for monsters all the time.