**Edit**
A double checked my math again after typing out this response, and now realize the mistake I made. I misread their multiattack as being Longsword x2 and Whip x1, when it should be just one longsword attack. I'm not going to adjust the rest of the post, in case anyone is curious, but my DPR value is about 20% to high as a result.
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This might be playstyle variance but I find that estimate implausibly high. Even if it somehow manages to hit with all of its attacks that's still only 60ish damage per round,
The sim I wrote doesn't bother with to hit values in DPR. It's purpose is to evaluate damage potential, not try and account for target AC's. Especially with a +14 to hit... But, a Balor that hits with all it's melee attacks is doing about 95.5 damage per round.
so it would have to be inflicting flame damage on the PCs eight times per round, which will only ever happen in an extremely melee-heavy party who are all consistently hitting the Balor every round (eight hits probably means about three attacks per turn from about four melee-only PCs), which means that the Balor is about to die messily anyway.
My estimate of hitting 4 targets per round with Flame Aura assumed 2 melee fighters that each hit the fiend twice. Remember, Flame Aura triggers both when someone strikes it, and for just being next to it when the Balor starts its turn. So that's generally going to be 3 triggers per person, or 6 triggers each round. Each trigger is 3d6 damage, for an average 63 damage per round of just the Flame Aura.
Are you sure you really meant 141 DPR? Your assumptions above (four flame targets per round) would yield a DPR of only around 100, which still seems high but less implausibly so.
I don't have the sim results saved (it's literally just a javascript-backed Google Sheets document I built to help me evaluate CRs for monsters), but I'll explain a bit of the math behind it. Keep in mind that it's not meant to simulate all the variances of combat or tactics or random swings in luck.
1. All damage is averaged.
2. Damage is evaluated for 40 rounds of combat. For monsters, I do this because it makes it easy to assume crit rates (2 crits per attack), and I can easily calculate how often something like a Breath Weapon recharges. For the Balor, this meant that out of the 120 attacks it makes over the 40 rounds, 114 deal normal damage, and 6 deal critical damage.
3. I then add damage modifiers that don't get multiplied with a crit, such as ability mods.
4. Damage totals are then divided by 40 to arrive at an average DPR.
In the Balor's case, it has 3 attacks plus the flame aura. The attacks do 6d8/6d8/5d6 and the flame aura is calculated separately (as above). Each hits 38 times and crits 2 times. The hits do 1,026/1,026/665 and the crits (Longsword crits are x3 rather than the normal x2) do 81/81/35. Flame Aura is doing 63 per round, or 2,520 in total. The only damage modifiers that apply now are the Balors' STR bonus of +8 for all 140 attacks, for a total of 1,120. Add all this up and you get 6,554 total damage over 40 rounds of combat. Which comes out to 163.85 DPR.
That total is actually higher than my first post of 141 DPR, and the reason is because my original sim forgot to factor the the 2 passive triggers of Flame Aura (ironic, I know).
Anyway, keep in mind that my sim is meant to be a balancing tool for a GM to evaluate the combat strength of a monster. I have another one written for character classes that goes further and actually accounts for hit chance against all AC's from 10-26, but that's not what this topic really called for.
Having said that, I can plainly see now that I need to update it to factor in just as many auto-misses (from rolling a 1) as it does crits, because the current build does not do it. Factoring the 6 misses into it lowers the DPR to 159.4.
The omission is easily corrected though, as are their weapon immunities (Balor should fear cold iron) and the Pit Fiend's lack of regeneration.
