Is this addition to the spell Negative Energy Flood balanced?

Question

In this question I outlined an inconsistency in the RAW interpretation of the spell Negative Energy Flood (XGtE, pg. 163): a size Large or larger creature killed by it rises as a size Medium zombie. To remedy this, I have added to the spell description to make it scale with the size of creature killed. Bold in the spell description indicates my addition to the spell.

Negative Energy Flood

5th-level Necromancy
Casting Time: 1 Action
Range: 60 feet
Components: V, M (a broken bone and a square of black silk)
Duration: Instantaneous

You send ribbons of negative energy at one creature you can see within range. Unless the target is undead, it must make a Constitution saving throw, taking 5d12 necrotic damage on a failed save, or half as much damage on a successful one. A target killed by this damage rises up as a zombie at the start of your next turn, having the same size and shape as the target. If the zombie is of size large or larger, it deals extra dice of damage on a hit. Double the weapon dice if the zombie is Large, triple the weapon dice if it’s Huge, and quadruple the weapon dice if it’s Gargantuan. Statistics for the zombie are in the Monster Manual. The zombie pursues whatever creature it can see that is closest to it.

If you target an undead with this spell, the target doesn’t make a saving throw. Instead, roll 5d12. The target gains half the total as temporary hit points.

This solves the obvious problem of Gargantuan creatures becoming Medium zombies, and I anticipate a size change alone has little implications for balance - correct me if I am wrong.

Potential balancing issues arise in increasing the damage output for Large and larger zombies. I borrowed the language directly from DMG pg. 278 in the section on creating your own monster stat blocks, taking into account the size of the monster. Step 11 of Creating a Monster Stat Block reads:

Big monsters typically wield oversized weapons that deal extra dice of damage on a hit. Double the weapon dice if the creature is Large, triple the weapon dice if it’s Huge, and quadruple the weapon dice if it’s Gargantuan.

A zombie has a Slam attack that deals 1d6+1 damage on a hit (Monster Manual, pg. 316). So for a large zombie, the damage on a hit would be 2d6+1; Huge, 3d6+1; and Gargantuan, 4d6+1.

I don't think only scaling the damage dice by size would make the spell too much more powerful than it already is, the zombie still has an AC of 8 and 22 hitpoints. What would start to tip the scales out of balance, I think, would be scaling the hit dice according to Step 8 in the same section of the DMG (pg. 276), especially for Gargantuan creatures. Scaling hitpoints in this way gives: Large - 25 (3d10+9), Huge - 28 (3d12+9), and Gargantuan - 40 (3d20+9).

Allowing the spell to scale hitpoints in this way for Gargantuan creatures gives the zombie nearly twice the staying power as the original version of the spell did. I also couldn’t think of a clear way to word it into the spell description. For this reason, I propose only scaling the damage dice for the zombie with the zombie's size.

Is this buff to Negative Energy Flood still appropriately balanced as a 5th-level spell?

Answer 1

This isn't unbalanced, exactly. The zombie that gets created isn't very good, and isn't even under your control!

We could compare this spell to animate objects, also a fifth-level wizard spell, which could create a huge animated object with AC10, 80hp, +8 to hit, and 17 damage. (It's a little better than this, because the spell creates some smaller animated objects too. Note also that the creature is under your control!) Your spell, even with a gargantuan zombie, has AC8, up to 40hp, +3 to hit, and 15 damage.

(Of course, your spell deals damage as well. negative energy flood deals average 32.5 damage to one target; we can compare to cone of cold which deals average 36 damage in an area. If we think having only one target makes the spell half as good as cone of cold, then we might expect negative energy flood to also create a creature that is half as good as conjure elemental, and then it would be "balanced" as two halves of other spells.)

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But here's my concern: the changes you have made are still not "realistic".

What if you cast this spell on a flying creature? You'd get a zombie with wings; can the zombie fly?

What if you cast it on a swimming creature? Do you get a zombie that can swim?

What if you cast it on an ooze? What if you cast it on a fire elemental?

In D&D 3.5e, they solved this problem by making "zombie" into a template which could be applied to "any corporeal creature with a skeletal system".

If you really want to pursue realism in zombie-animation, it seems to me that what you have here doesn't go far enough. You're going to want to create something like the 3.5e zombie template, which tells you what happens when any creature turns into a zombie. Unfortunately that's a lot of additional rules complexity.

(And then, once you have all those rules, you have to worry that someone will publish a weird monster in a sourcebook five years from now, and when combined with your zombie template it will be totally broken...)

I speculate that the designers of 5e considered this problem, and they said: "nope, that's too complicated," and they declared that there was just one standard zombie and that was all the rules they were writing.

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The 5e approach to this sort of thing seems to be to just not specify any rules and leave it open to DM fiat what happens. If we wanted to take that approach, we'd tack on a line like this to the spell, to remind the reader that Rule 0 can be applied here:

A target killed by this spell rises up as a zombie at the start of your next turn. The DM may choose to use the stats for the zombie on page 316 of the Monster Manual, or, if the target was not a medium-sized humanoid, the DM may improvise other stats at their discretion.

I actually think that would be pretty reasonable.