Hey amigo, thanks for the feedback.
On Mutagenic damage, I read the question as dealing with the secondary Mutagenic damage caused by the growths in each following round.
Assume Super Handsome Dude gets hit with a single target use of Mutagenic damage as you specify above, dealing 1000 damage. A "hostile mass of sentient tumorous eyes, teeth and tendrils" erupts in Super Handsome Dude's perfectly sculpted left bicep. The next turn, this tumorous mass (Mass A, 1000hp) makes a single attack (the sidebar doesn't specify an attack modifier, so presumably this is an automatic hit?)
I think the simplest method for a Ranged Spittle Attack is: Growth's Attack Bonus = Target's Proficiency Bonus x2.
To simply attack the host an automatic hit probably makes the most sense.
and deals 50d6 mutagenic damage (175 average, or possibly 300 max damage - I don't know enough about your rules on damage at this level of play).
It would just take 50d6 unless Vulnerable to Acid.
As a result of the additional Mutagenic damage, does (i) Super Handsome Dude get a new playfriend erupting in his amazing jawline (Mass B(A), 175hp, 1/rnd deals 8d6 Mutagenic damage)? Or (ii) does Mass A grow in size by 175hp and in damage by 8d6 to 1175hp and 58d6 respectively? (My notation [Mass B(A)] means that the Mass B arises from damage from A.)
Assuming (i) above, then the following round would give Mass C(A), of equivalent hp and damage potential as Mass B(A), as well as Mass D(B), a 28hp, 1d6/rnd damaging mass.
The following round would give 4 new masses would arise, but only 3 of these would be viable to deal any damage (particularly with Damage Threshold in play). In other words, Mass D(B) can be treated as functionally not causing any new masses. Some of the damage potential is watered down overall, but each instance has to be dealt with separately to remove.
If instead (ii) above, then Mass A scales damage indefinitely without watering down the damage potential.
Combat lasting enough rounds for there to be any meaningful difference in damage is unlikely, right? A single, expanding growth is easier for the DM to run, but also requires only a single action (or whatever aspect of the action economy that applies to play at these levels to cast Heal) to remove. In the meanwhile, one must also ask whether Super Handsome Dude's tumorous stowaways somehow manage to only make him look even better.
Design and tradeoffs at play again.
My initial suggestion was that single target attacks create new Growths while area of effect attacks adds to an existing growth - which itself raises the question if you have two growths does the Mutagenic damage affect all equally.
I think for simplicity, having one growth at a time probably makes things easier.
Other Minor Rules Tweaks/Fixes:
Vulnerability. Its now +50%, with Super-Vulnerability at +100%. Further testing just saw the numbers spiking too high.
Energy Immunity means you get its opposed Energy Vulnerability (+50%) Fire vs Ice etc.
Energy Absorption means you get its opposed Super-Vulnerability (+100%)
Currently going back and forth fixing the Neutronium Form Template (in the Boons) and the other 25 templates to make sure they are balanced...or at least as balanced as these things can be.
Critical Multipliers: I have also capped Critical Damage Multipliers to one Source so Mjolnir will only be dealing x5 damage, not x7 or whatever. Again, it was way to easy to spike. FWIW Critical Chance Modifiers stack.
Attack Conditions: Also been testing if attacks which inflict a Condition should:
A. Only apply to targets that are Bloodied
B. If you use Legendary Resistance to succeed a failed save you are Immune to that Condition for the rest of the round.
This should alleviate Stun-locking and so forth, especially with so many attacks potentially going around.
