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
Casting Time: 1 Action
Range: 60 feet
Components: V, M (a broken bone and a square of black silk)
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?